// ── The de-entanglement registries ─────────────────────────────────────────
//
// Phase 2, PR 4 of docs/website/MODULE_SYSTEM.md §2.7 — the three seams §1.8 and
// §1.9 identified, where core code and game-specific content are tangled in one
// file and a folder move cannot separate them. The normative contract is
// docs/website/MODULE_API.md §2.4.
//
// The three:
//
//   1. `registerExtension(slot, router)`   — §1.9. Module routes hanging off a
//      CORE resource (`/admin/users/:id`), so all six shard sub-paths keep their
//      URLs while core never learns what "shard" means.
//   2. `registerNotificationStreams(streams)` — §1.8. The push-stream catalog:
//      push INFRASTRUCTURE is core, this CATALOG is content.
//   3. `registerAnnounceLeg({ leg, label, dispatch, classify })` — §1.8. The news
//      dispatcher's delivery legs; Discord is core, town crier is content.
//
// **Core registers through these functions too, and is the only registrant until
// Phase 3.** `registerCore()` below is called explicitly from app.js before
// `modules.load()` — explicit, never lazy, the same decision the loader's trigger
// took (MODULE_API.md §7.6). Core going through the same door is the point: a
// registry only core's hardcoded base bypasses is a registry whose first real
// exercise is a module, which is the drift this PR exists to prevent.
//
// **Registering is validate-then-commit, per registrant.** `apply()` checks every
// claim in a batch before it writes any of them, so a module that registers two
// streams and then throws — or fails a later validation step in the loader — has
// left nothing behind. That is the registry-side twin of the loader's second-pass
// mount rule: nothing a module claims takes effect until the module as a whole is
// known good.
//
// Two more arrived with the engagement system (ENGAGEMENT.md Phase 2), from a
// different workstream but through the same door:
//
//   4. `registerEventTriggers(triggers)` — §4.3. The payload CONTRACT behind an
//      event id: what a template may interpolate, and how widely a rule may
//      ever send it (the ceiling, G24).
//   5. `registerAudiences(audiences)`    — §5.1a. Named sets of user ids a
//      module can resolve over its own data, for an operator to point a rule at.
//
// And a sixth, in Phase 11b (decision 7):
//
//   6. `registerEngagementSeeds({ templates, ruleGroups })` — the message BODIES
//      and the shipped rules behind 4 and 5. A module declaring a trigger could
//      say what its payload was and never say what it should read like, so a
//      module's mail was core's generic body or nothing.
//
// **6 stores data and nothing else — no function, no handle.** A template is
// blocks and a rule is columns, both validated here and both written by core's
// own seeders (`engagement/moduleSeeds.js`), which is what keeps `seed_version`,
// `customized` and the block registry in the one file that owns them. It is
// emphatically not a send path: a module still cannot mail anyone (§1.2).
//
// **Triggers and notification streams share ONE id namespace** (the org lead's
// §7.2 decision). A stream entry is a subscription toggle and a trigger is a
// payload contract, so they stay two REGISTRATIONS with two shapes — but an id
// has exactly one owner across both, and `news.post` names one event whichever
// question is being asked of it. See the cross-facet checks in `apply()`.
//
// Nothing here reaches the database or the network. It is a require-time-safe
// collection of what core and modules have declared, read at request time.

const express = require('express')

const log = require('../utils/logger')('modules')
const ceilings = require('./ceilings')

// ── State ──────────────────────────────────────────────────────────────────

// slot → { router, filledBy }. `router` is created when CORE DECLARES the slot
// and mounted immediately; registrants `use()` into it later. That indirection is
// not optional: users.router.js is required while app.js is being built, long
// before any module has been scanned, so the thing core mounts has to be a stable
// object that can still be empty.
const slots = new Map()

// Registration order, which is display order in the app's notifications screen.
const streams = []
const streamOwners = new Map() // stream id → owner id, for the collision message

// leg id → { owner, leg, label, dispatch, classify }
const legs = new Map()

// owner -> { onSaved?, onDeleted? }. Post hooks (§1.8, API 1.1.0). A Map keyed by
// owner rather than a flat list, so a registrant is a single subscription that
// can be reported and reasoned about as one thing — and so registering twice is
// a collision with a name attached rather than a silently doubled side effect.
const postHooks = new Map()

// { owner, getTeams, getTeamMembers, getTeamLeaders } or null — the Team provider
// (API 1.6.0, TEAMS.md §2.3).
//
// A SINGLE value rather than a Map, unlike every registry above it, and that is
// the contract: one provider per deployment. Teams have one authoritative source
// by construction — two modules answering "what teams exist" would produce two
// disjoint sets under one `teams` table with no rule for merging them, so a
// second registration is a collision rather than an addition.
let teamProvider = null

// command name → { owner, name, description, options, access, handler }. Slash
// commands a registrant has published for the chat platform (API 1.6.0,
// TEAMS.md §7.1).
//
// The DEFINITION and the HANDLER are registered together and the handler runs
// HERE, in the website process; the bot pulls the definitions over the internal
// API and owns every Discord-specific concern. That split is forced — the bot
// container has no `modules` volume, so a module physically cannot put a handler
// in it (§0.4) — and it is also the boundary we would pick anyway: a module
// calling `interaction.deferReply()` would be a module holding a Discord handle.
const slashCommands = new Map()

// trigger id → { owner, id, label, description, kind, subjectKey, audience,
// ceiling, version, variables } (ENGAGEMENT.md §4.3, API 1.7.0).
//
// A Map rather than an array, unlike `streams`: a stream catalog is READ WHOLE
// (the app renders it in registration order) and a trigger is READ BY ID (the
// emit path, the rule editor, the template editor), so insertion order is kept
// for display and the lookup is the primary access.
const triggers = new Map()

// audience id → { owner, id, label, description, params, ceiling, resolve }
// (§5.1a). Its own id space, not the trigger/stream one: an audience names a set
// of PEOPLE and a trigger names an EVENT, and `uo.team.members` colliding with a
// trigger of the same name would be a collision between two unrelated things.
const audiences = new Map()

// owner → { templates: [...], ruleGroups: [...] } (ENGAGEMENT.md Phase 11b,
// decision 7). What a module ships as CONTENT rather than as contract: the
// bodies its triggers render through, and the rules an operator switches on.
//
// Keyed by owner and not by template key, because the seeder runs per module —
// a module the operator disabled is skipped whole, and a module that failed to
// load never gets here at all.
const engagementSeeds = new Map()

let coreRegistered = false

// Stream ids that predate the module system and may not carry their owner's
// prefix — the exact counterpart of the loader's LEGACY_TABLE_PREFIXES, for the
// exact same reason. These seven ids are stored in `notification_subs` rows and
// are read by a shipped Android client; renaming them in Phase 3 would be a data
// migration and a client break, so `uo` keeps them and the prefix rule stays real
// for every module written after it.
const LEGACY_STREAM_IDS = {
  uo: [
    'server.status', 'idoc.warning', 'champ.start', 'governor.election',
    'vendor.sale', 'house.idoc', 'account.login',
  ],
}

// Likewise for announce legs: `towncrier` is a stored value in
// announce_job_legs.leg and the body of the admin retry endpoint.
const LEGACY_LEGS = { uo: ['towncrier'] }

// ONE grammar for the one namespace streams and triggers share. It relaxes what
// `STREAM_ID` used to allow by admitting `_` inside a segment, because the
// trigger ids this contract is written for have them (`uo.house.idoc_warning`,
// ENGAGEMENT.md §4.3) and two grammars over one namespace would mean an id that
// is legal as a trigger and illegal as the stream it is the same event as.
// Relaxation only: every id valid before is valid now, and no stored id changes.
const EVENT_ID = /^[a-z][a-z0-9_]*(\.[a-z][a-z0-9_]*)+$/
const LEG_ID = /^[a-z][a-z0-9.]{1,62}$/
// Audiences are their own id space (see the `audiences` Map), so they get their
// own constant even though the grammar is the same one.
const AUDIENCE_ID = EVENT_ID

// A module's claim must carry its id. Core's ids are its own namespace, and the
// grandfathered names are the ones that predate all of this.
function namespaced(owner, name, legacy) {
  return owner === 'core' || name.startsWith(`${owner}.`) || (legacy[owner] || []).includes(name)
}

// ── Extension slots (§1.9) ─────────────────────────────────────────────────

/**
 * Core declares an extension slot and gets the router to mount for it.
 *
 * ONLY core may declare a slot; a module may only fill one (MODULE_API.md §2.4).
 * That asymmetry is why this is not on the `api` object handed to a module.
 *
 * `mergeParams` so the slot's router sees the parent's `:id`. Core's own routes
 * on the resource are declared before the slot is mounted, so first-match-wins
 * gives core the path conflict, as the contract requires.
 *
 * @returns {import('express').Router} mount this at the resource, once.
 */
function declareSlot(slot) {
  if (slots.has(slot)) throw new Error(`extension slot "${slot}" already declared`)
  const router = express.Router({ mergeParams: true })
  slots.set(slot, { router, filledBy: null })
  return router
}

/** Does this slot exist? The loader asks, to validate `extensions` in a manifest. */
const hasSlot = (slot) => slots.has(slot)

/** Who filled a slot, or null. */
const slotFilledBy = (slot) => (slots.get(slot) || {}).filledBy || null

/**
 * Every FILLED slot, for the OpenAPI build step (swagger/slotSpecs.js).
 *
 * `router` is the slot's own stable router — the object mounted on the resource —
 * so the build can find it in the live express stack and recover the prefix it
 * hangs at without a hardcoded table.
 */
const filledSlots = () =>
  [...slots.entries()]
    .filter(([, e]) => e.filledBy)
    .map(([slot, e]) => ({ slot, filledBy: e.filledBy, router: e.router, specFile: e.specFile || null }))

/**
 * A DECLARED slot's stable router, filled or not.
 *
 * `filledSlots()` answers what the build needs — a filled slot has a spec file
 * to generate a fragment from. This answers what a test needs: the slot exists
 * from the moment core declares it at require time, and its position in the
 * express stack has to stay findable whether or not a module has filled it.
 * Before Phase 3 the two questions had the same answer, because core filled the
 * only slot itself.
 */
const declaredSlotRouter = (slot) => (slots.get(slot) || {}).router || null

// ── Notification streams (§1.8) ────────────────────────────────────────────

/** The whole catalog, core's entries first, in registration order. */
const allStreams = () => streams.slice()

/** Is this a stream anyone registered? Gates a subscription write. */
const isValidStream = (id) => streamOwners.has(id)

/** Ids of the owner-keyed streams — those needing a linked game account. */
const personalStreams = () => new Set(streams.filter((s) => s.personal).map((s) => s.id))

// ── Post hooks (§1.8) ──────────────────────────────────────────────────────

// Core's CMS is the only writer of posts, and a module may need to mirror one
// somewhere core knows nothing about — module-uo keeps UO's in-game Town Cryer
// News gump in step with it. Before this existed, core's post controller
// required `utils/newsGump` directly, which is precisely the coupling the
// extraction had to remove: core's publish path naming a UO file.
//
// It is deliberately NOT folded into `registerAnnounceLeg`, which fires on the
// same transition. A leg is a one-shot DELIVERY with retry and classification;
// a post hook maintains idempotent STATE, has to run on delete as well as save,
// and refreshes silently on an edit. Overloading the leg would have meant a
// dispatch that must not be retried and a classify that means nothing.

/** Every registered hook, in registration order. */
const postHookEntries = () => [...postHooks.entries()].map(([owner, h]) => ({ owner, ...h }))

/**
 * Fire `event` at every registered hook, one at a time, never throwing.
 *
 * Best-effort by contract, and awaited rather than fired-and-forgotten: core's
 * own call site awaited `newsGump.syncPost` before this existed, so a save that
 * returns 200 still means the mirror was attempted. One subscriber's failure
 * must not cost another's, and none of them may cost the save — a sidecar
 * hiccup breaking a post edit would be a worse bug than a stale gump.
 */
async function dispatchPostHook(event, payload) {
  for (const { owner, [event]: fn } of postHookEntries()) {
    if (typeof fn !== 'function') continue
    try {
      await fn(payload)
    } catch (err) {
      log.warn('post hook failed', { owner, event, message: err.message })
    }
  }
}

// ── Announce legs (§1.8) ───────────────────────────────────────────────────

/** Every registered leg, in registration order. */
const announceLegs = () => [...legs.values()]

/** Just the ids — the enqueue order and the retry endpoint's allowlist. */
const announceLegIds = () => [...legs.keys()]

/** One leg, or null. */
const announceLeg = (leg) => legs.get(leg) || null

// ── Team provider (TEAMS.md §2.3) ──────────────────────────────────────────

/** The registered provider, or null when no module supplies one. */
const registeredTeamProvider = () => teamProvider

/** Is there a Team provider at all? Read by the reconciler and the read API. */
const hasTeamProvider = () => teamProvider !== null

// ── Slash commands (TEAMS.md §7.1) ─────────────────────────────────────────

/**
 * Every registered command WITHOUT its handler — what `/internal/commands`
 * serves to the bot.
 *
 * The handler is stripped rather than merely un-serialisable-and-ignored: this
 * is the object that crosses a process boundary, and the definition half is the
 * whole of what the bot is allowed to know. `owner` rides along so the bot can
 * name the module in a collision warning.
 */
const slashCommandDefinitions = () =>
  [...slashCommands.values()].map(({ handler, ...definition }) => definition)

/** One command, handler included. The dispatcher's lookup. */
const slashCommand = (name) => slashCommands.get(name) || null

// ── Event triggers (ENGAGEMENT.md §4.3) ────────────────────────────────────

/** Every declaration, core's first, in registration order. The admin catalog. */
const allTriggers = () => [...triggers.values()]

/** One declaration, or null. The emit path's lookup and the rule editor's. */
const eventTrigger = (id) => triggers.get(id) || null

/**
 * Who owns this id, across BOTH facets — the one-namespace question.
 *
 * A caller asking "may this module emit this?" wants this rather than
 * `eventTrigger(id).owner`, because an id can be held as a stream by one owner
 * and not yet declared as a trigger by anyone, and that id is still taken.
 */
const eventOwner = (id) => triggers.get(id)?.owner || streamOwners.get(id) || null

// ── Audiences (§5.1a) ──────────────────────────────────────────────────────

/**
 * Every declaration WITHOUT its resolver — what the admin surface serves.
 *
 * The resolver is stripped for the same reason a slash command's handler is:
 * this is the object that leaves the process, and `resolve` is a function over a
 * module's own store that no client has any business holding a reference to.
 */
const allAudiences = () => [...audiences.values()].map(({ resolve, ...rest }) => rest)

/** One declaration, resolver included. The engine's lookup. */
const audience = (id) => audiences.get(id) || null

/**
 * Resolve a declared audience to user ids, never throwing.
 *
 * Three answers, and the middle one is the contract (§5.1a rule 4): a registered
 * audience answers `{ dormant: false, userIds }`; an audience whose module is
 * uninstalled answers `{ dormant: true, userIds: [] }` — the EMPTY set and a
 * flag, never an error and never a fallback to some other set of people; and a
 * resolver that throws or answers a non-array is logged and treated as empty,
 * because a module's storage problem must not become a send to the wrong people.
 *
 * `userIds` is filtered to positive integers here rather than trusted. It is the
 * one value a module hands core that decides who receives mail, and the resolver
 * is module code running over a module's own store.
 */
async function resolveAudience(id, params = {}) {
  const entry = audiences.get(id)
  if (!entry) return { dormant: true, userIds: [] }
  try {
    const raw = await entry.resolve(params)
    if (!Array.isArray(raw)) {
      log.warn('audience resolver did not return an array', { audience: id, owner: entry.owner })
      return { dormant: false, userIds: [] }
    }
    const userIds = [...new Set(raw.map(Number).filter((n) => Number.isInteger(n) && n > 0))]
    return { dormant: false, userIds }
  } catch (err) {
    log.error('audience resolver failed', { audience: id, owner: entry.owner, message: err.message })
    return { dormant: false, userIds: [] }
  }
}

// ── Shape checks, run the moment a registrant calls ────────────────────────
//
// Split from the collision checks below on the same line PR 3 drew through
// schema-fragment validation: what can be decided from the argument alone is
// decided AT THE CALL, so the error carries the registrant's own stack. What
// depends on other registrants has to wait for the batch to be complete.

function checkStreamShape(entry) {
  if (!entry || !EVENT_ID.test(entry.id || '')) {
    throw new Error(`registerNotificationStreams: bad stream id "${entry && entry.id}"`)
  }
  if (!entry.label) throw new Error(`registerNotificationStreams: stream "${entry.id}" has no label`)
  return {
    id: entry.id,
    label: entry.label,
    description: entry.description || '',
    personal: Boolean(entry.personal),
    requiresLinkedAccount: Boolean(entry.requiresLinkedAccount),
  }
}

function checkLegShape(entry) {
  const { leg, label, dispatch, classify } = entry || {}
  if (!LEG_ID.test(leg || '')) throw new Error(`registerAnnounceLeg: bad leg id "${leg}"`)
  if (typeof dispatch !== 'function') throw new Error(`announce leg "${leg}" has no dispatch()`)
  if (typeof classify !== 'function') throw new Error(`announce leg "${leg}" has no classify()`)
  return { leg, label: label || leg, dispatch, classify }
}

// Three methods are REQUIRED, with no optional half. A provider that could list
// Teams but not their members would leave core holding Teams it can never
// populate, and the reconciler has no sensible behaviour for that — it is not the
// same as a call that fails, which is staleness and already handled (§2.4). A
// module unable to answer one of the three answers `{ ok: false }` at call time.
//
// `projectRoster` is the fourth and is OPTIONAL (TEAMS.md §3.3): it expresses an
// audience model, and a module with no rung system of its own has no opinion to
// express. Omitting it means core serves rosters at its own public shape;
// implementing it means core fails CLOSED when the call cannot be made, so this
// is a member to add deliberately rather than by habit.
//
// `pageUrlTemplate` is the fifth, also OPTIONAL, and is data rather than a method
// — see its own comment below. A module that omits it costs its deployment
// clickable links in Team notification email and nothing else.
//
// The copy is explicit rather than a spread: this object is what core calls, so
// anything not named here is not part of the contract and must not survive
// registration. A method that silently rode along would look implemented from the
// module's side and be invisible from core's.
function checkTeamProviderShape(entry) {
  const provider = entry || {}
  const out = {}
  for (const name of ['getTeams', 'getTeamMembers', 'getTeamLeaders']) {
    if (typeof provider[name] !== 'function') {
      throw new Error(`registerTeamProvider: ${name}() is missing or not a function`)
    }
    out[name] = provider[name]
  }
  if (provider.projectRoster !== undefined) {
    if (typeof provider.projectRoster !== 'function') {
      throw new Error('registerTeamProvider: projectRoster must be a function if present')
    }
    out.projectRoster = provider.projectRoster
  }
  if (provider.pageUrlTemplate !== undefined) {
    out.pageUrlTemplate = checkPageUrlTemplate(provider.pageUrlTemplate)
  }
  return out
}

// `pageUrlTemplate` is the fifth member and OPTIONAL (TEAMS.md §6.4, phase 6).
//
// **Why a module has to supply this at all.** Teams are a contract primitive with
// no core surface: core owns the tables and the access rules, and the MODULE owns
// the page, because core does not own the word for a Team. That is settled and
// right — but it leaves core unable to write a link to one, and a notification
// email that cannot link to the thread it is about is most of the way to useless.
// So the module that owns the page says where it is.
//
// **A template, not a callback.** Core substitutes `{externalId}` and `{slug}`
// into a relative path and does nothing else with it. A function would be a
// module hook on the mail path — one more thing that can hang or throw between a
// forum reply and the mail about it — to produce a string that never varies.
//
// Validated hard, because the output goes into an email as a link. Relative only:
// a template naming its own host would let a module redirect the site's outbound
// mail somewhere else, and there is no reason for one to.
// One leading slash, and the second character may not be another. `//evil.test/x`
// passes an "is it rooted" check and is a PROTOCOL-RELATIVE url — core prefixing
// its own base makes it harmless today, but a template is a string that ends up
// in an href sooner or later, and this is a character class rather than a
// judgement call about who concatenates it.
const PAGE_URL_TEMPLATE = /^\/(?!\/)[A-Za-z0-9\-._~/{}]*$/

function checkPageUrlTemplate(value) {
  if (typeof value !== 'string' || !PAGE_URL_TEMPLATE.test(value)) {
    throw new Error(`registerTeamProvider: pageUrlTemplate must be a relative path, got "${value}"`)
  }
  return value
}

// A slash command's name and description are validated HERE and not only at the
// bot, for a reason worth stating: the bot registers the whole set in a single
// `REST.put(applicationGuildCommands)`, so ONE malformed definition is rejected
// by Discord as a batch and takes every other command down with it — including
// the bot's own. A definition that cannot be registered must therefore fail at
// `register()`, where it belongs to a module that can be named and marked
// failed, rather than at the next `ready` where it looks like the bot is broken.
//
// **Commands are NOT namespaced under their owner, unlike every other id in this
// file.** Discord's name grammar has no `.` in it, so `uo.guild` is unregistrable
// and the prefix rule cannot be expressed. Collisions are caught by first-come
// instead, with the holder named — and the bot resolves the one collision core
// cannot see (a pulled name against its own built-ins) in the module's disfavour.
const SLASH_NAME = /^[a-z0-9_-]{1,32}$/
const SLASH_ACCESS = ['everyone', 'linked', 'staff']

// §7.1.1: `string | integer | boolean | user`, and deliberately nothing else. No
// subcommand groups, autocomplete, attachments, modals or component
// interactions. Those are exactly the features whose semantics do not survive a
// second platform, and admitting one here is how Discord specifics leak into a
// platform-agnostic registration API by accident.
const SLASH_OPTION_TYPES = ['string', 'integer', 'boolean', 'user']

function checkSlashOption(command, option) {
  const { name, type, description, required, choices } = option || {}
  const where = `registerSlashCommands: ${command}`
  if (!SLASH_NAME.test(name || '')) throw new Error(`${where}: bad option name "${name}"`)
  if (!SLASH_OPTION_TYPES.includes(type)) {
    throw new Error(`${where}: option "${name}" has unsupported type "${type}" (§7.1.1)`)
  }
  if (!description || description.length > 100) {
    throw new Error(`${where}: option "${name}" needs a description of 1-100 characters`)
  }
  const out = { name, type, description, required: Boolean(required) }
  if (choices !== undefined) {
    if (!Array.isArray(choices) || !choices.length) {
      throw new Error(`${where}: option "${name}" has an empty choices list`)
    }
    // Only the two option types Discord itself allows choices on. `boolean` is
    // already a two-value choice and `user` is a picker; a choices list on
    // either is a misunderstanding worth failing rather than dropping.
    if (type !== 'string' && type !== 'integer') {
      throw new Error(`${where}: option "${name}" is ${type}; choices need string or integer`)
    }
    out.choices = choices.map((c) => {
      if (!c || !c.name || c.value === undefined) {
        throw new Error(`${where}: option "${name}" has a choice with no name/value`)
      }
      return { name: String(c.name), value: c.value }
    })
  }
  return out
}

/**
 * `registerSlashCommands([{ name, description, options, access, handler }])`.
 *
 * `access` is enforced TWICE and this copy is not the gate: the bot sets
 * Discord-side default member permissions from it where it can, and the
 * dispatcher re-checks it on every call. Client-side is about not advertising a
 * dead end; the server is the boundary — the same principle the nav follows.
 */
function checkSlashCommandShape(entry) {
  const { name, description, options, access, handler } = entry || {}
  if (!SLASH_NAME.test(name || '')) {
    throw new Error(`registerSlashCommands: bad command name "${name}" (lowercase, 1-32, no dots)`)
  }
  if (!description || description.length > 100) {
    throw new Error(`registerSlashCommands: ${name} needs a description of 1-100 characters`)
  }
  if (typeof handler !== 'function') throw new Error(`registerSlashCommands: ${name} has no handler()`)
  if (access !== undefined && !SLASH_ACCESS.includes(access)) {
    throw new Error(`registerSlashCommands: ${name} has unknown access "${access}"`)
  }
  if (options !== undefined && !Array.isArray(options)) {
    throw new Error(`registerSlashCommands: ${name} options must be an array`)
  }
  const checked = (options || []).map((o) => checkSlashOption(name, o))
  // Discord rejects a definition that puts an optional option before a required
  // one, and does it for the whole batch. Sorting silently would change what the
  // module wrote; this is the module's own ordering bug and it gets its name.
  const firstOptional = checked.findIndex((o) => !o.required)
  if (firstOptional !== -1 && checked.slice(firstOptional).some((o) => o.required)) {
    throw new Error(`registerSlashCommands: ${name} lists a required option after an optional one`)
  }
  return { name, description, options: checked, access: access || 'everyone', handler }
}

/**
 * `registerPostHook({ onSaved, onDeleted })` — both optional, at least one
 * required. A registration with neither is a subscription that can never fire,
 * which is a typo rather than an intention.
 */
function checkPostHookShape(entry) {
  const { onSaved, onDeleted } = entry || {}
  for (const [name, fn] of [['onSaved', onSaved], ['onDeleted', onDeleted]]) {
    if (fn !== undefined && typeof fn !== 'function') {
      throw new Error(`registerPostHook: ${name} must be a function`)
    }
  }
  if (!onSaved && !onDeleted) throw new Error('registerPostHook: needs onSaved or onDeleted')
  return { onSaved, onDeleted }
}

// ── Event trigger shape (ENGAGEMENT.md §4.3) ───────────────────────────────

// Deliberately small, and closed. A payload variable ends up interpolated into
// an email, so the set is "things a template can render and a preview can fake",
// not "things JSON can hold". No `object` and no `array`: a template that has to
// walk a structure is a template that has outgrown interpolation, and a block
// type is the right answer to that (§4.4).
const VARIABLE_TYPES = ['string', 'int', 'float', 'boolean', 'datetime', 'url']

// `event` fires from ctx.events.emit; `scheduled` is evaluated periodically and
// has no evaluator yet — the org lead's §7.1 Q6 answer is design now, build after
// Phase 9. It is declarable from today so `kind` is in the contract, the manifest
// and every stored declaration before there are rows to migrate.
const TRIGGER_KINDS = ['event', 'scheduled']

const VARIABLE_NAME = /^[a-z][A-Za-z0-9]{0,39}$/

function checkTriggerVariable(triggerId, entry, seen) {
  const { name, type, required, example, description } = entry || {}
  const where = `registerEventTriggers: ${triggerId}`
  if (!VARIABLE_NAME.test(name || '')) throw new Error(`${where}: bad variable name "${name}"`)
  if (seen.has(name)) throw new Error(`${where}: variable "${name}" declared twice`)
  seen.add(name)
  if (!VARIABLE_TYPES.includes(type)) {
    throw new Error(`${where}: variable "${name}" has unsupported type "${type}"`)
  }
  // REQUIRED, and the one field of this shape that looks optional and is not
  // (§4.3 property 3). Without an example, previewing or test-sending a template
  // needs a live game event — which is exactly how template systems come to be
  // shipped untested. It is cheap to write at declaration time and impossible to
  // reconstruct later.
  if (example === undefined || example === null || example === '') {
    throw new Error(`${where}: variable "${name}" needs an example (§4.3 — it is the preview)`)
  }
  return {
    name,
    type,
    required: Boolean(required),
    example,
    description: description || '',
  }
}

/**
 * `registerEventTriggers([{ id, label, kind, subjectKey, audience, ceiling, version, variables }])`.
 *
 * Everything decidable from the argument alone is decided here, at the call, so
 * the error carries the registrant's own stack. The one-namespace collision — is
 * this id already someone's stream? — depends on other registrants and waits for
 * `apply()`, exactly as a stream's own collision does.
 *
 * The copy is explicit rather than a spread, like `checkTeamProviderShape`: this
 * object is served to the admin UI and frozen into a committed manifest, so
 * anything not named here is not part of the contract and must not ride along.
 */
function checkTriggerShape(entry) {
  const t = entry || {}
  if (!EVENT_ID.test(t.id || '')) {
    throw new Error(`registerEventTriggers: bad trigger id "${t.id}"`)
  }
  if (!t.label) throw new Error(`registerEventTriggers: trigger "${t.id}" has no label`)

  const kind = t.kind || 'event'
  if (!TRIGGER_KINDS.includes(kind)) {
    throw new Error(`registerEventTriggers: ${t.id} has unknown kind "${t.kind}"`)
  }

  // G24. Required with no default — a ceiling that could be forgotten is a
  // ceiling that gets forgotten on the one trigger it mattered for, and there is
  // no safe value to guess: `owner` would silently break a broadcast and
  // `authenticated` would silently widen a staff-only event.
  if (!ceilings.isCeiling(t.ceiling)) {
    throw new Error(
      `registerEventTriggers: ${t.id} needs a ceiling, one of ${ceilings.CEILINGS.join(', ')}`,
    )
  }
  // The DEFAULT a rule is created with; the ceiling is the maximum it may be
  // raised to. Defaulting it to the ceiling is right — a trigger that declares no
  // opinion gets the widest it permits, and an operator narrows from there.
  const audienceDefault = t.audience || t.ceiling
  if (!ceilings.permits(t.ceiling, audienceDefault)) {
    throw new Error(
      `registerEventTriggers: ${t.id} default audience "${audienceDefault}" is not permitted by ceiling "${t.ceiling}"`,
    )
  }

  const version = t.version === undefined ? 1 : t.version
  if (!Number.isInteger(version) || version < 1) {
    throw new Error(`registerEventTriggers: ${t.id} has a bad version "${t.version}"`)
  }

  if (t.variables !== undefined && !Array.isArray(t.variables)) {
    throw new Error(`registerEventTriggers: ${t.id} variables must be an array`)
  }
  const seen = new Set()
  const variables = (t.variables || []).map((v) => checkTriggerVariable(t.id, v, seen))

  // A subjectKey naming a variable that does not exist would produce a cooldown
  // keyed on `undefined` — i.e. one cooldown for every subject at once, which
  // looks like the feature working until the day two houses share it (§4.1).
  if (t.subjectKey !== undefined && !seen.has(t.subjectKey)) {
    throw new Error(
      `registerEventTriggers: ${t.id} subjectKey "${t.subjectKey}" is not one of its variables`,
    )
  }

  return {
    id: t.id,
    label: t.label,
    description: t.description || '',
    kind,
    subjectKey: t.subjectKey === undefined ? null : t.subjectKey,
    audience: audienceDefault,
    ceiling: t.ceiling,
    version,
    variables,
  }
}

// ── Audience shape (§5.1a) ─────────────────────────────────────────────────

// Two types, and no more. A param is something an operator types into a rule
// editor to point a declared audience at one row of a module's data ("which
// Team?"), so it is an identifier or a word. Anything richer is a query, and a
// query surface is the free-form list building Q7 rules out.
const AUDIENCE_PARAM_TYPES = ['int', 'string']

function checkAudienceParam(audienceId, entry, seen) {
  const { id, type, required, label } = entry || {}
  const where = `registerAudiences: ${audienceId}`
  if (!VARIABLE_NAME.test(id || '')) throw new Error(`${where}: bad param id "${id}"`)
  if (seen.has(id)) throw new Error(`${where}: param "${id}" declared twice`)
  seen.add(id)
  if (!AUDIENCE_PARAM_TYPES.includes(type)) {
    throw new Error(`${where}: param "${id}" has unsupported type "${type}"`)
  }
  return { id, type, required: Boolean(required), label: label || id }
}

/**
 * `registerAudiences([{ id, label, description, params, ceiling, resolve }])`.
 *
 * The resolver returns USER IDS and nothing else (§5.1a rule 2). It is not handed
 * a template, a channel or an address and it cannot enumerate them — a module
 * still cannot send mail, and this must not become the back door that lets it.
 * Core maps ids to addresses on its own side, after preferences, suppression and
 * the verification gate.
 */
function checkAudienceShape(entry) {
  const a = entry || {}
  if (!AUDIENCE_ID.test(a.id || '')) throw new Error(`registerAudiences: bad audience id "${a.id}"`)
  if (!a.label) throw new Error(`registerAudiences: audience "${a.id}" has no label`)
  if (!ceilings.isCeiling(a.ceiling)) {
    throw new Error(
      `registerAudiences: ${a.id} needs a ceiling, one of ${ceilings.CEILINGS.join(', ')}`,
    )
  }
  if (typeof a.resolve !== 'function') throw new Error(`registerAudiences: ${a.id} has no resolve()`)
  if (a.params !== undefined && !Array.isArray(a.params)) {
    throw new Error(`registerAudiences: ${a.id} params must be an array`)
  }
  const seen = new Set()
  const params = (a.params || []).map((p) => checkAudienceParam(a.id, p, seen))
  return {
    id: a.id,
    label: a.label,
    description: a.description || '',
    params,
    ceiling: a.ceiling,
    resolve: a.resolve,
  }
}

// ── Engagement seeds (Phase 11b, decision 7) ───────────────────────────────
//
// **Two mechanisms, and the asymmetry between them is the whole design.**
//
// A TEMPLATE is re-ensured on every boot. Its row carries `seed_key`,
// `seed_version` and `customized`, so re-ensuring is how a better default
// reaches a deployment without stealing an operator's edit (§4.6.1 property 3),
// and a template added in a later module version reaches every deployment rather
// than only fresh ones.
//
// A RULE is the opposite. Re-ensuring one would resurrect a rule an operator
// deleted and reset one they enabled — so rules arrive in named GROUPS, each
// with its own one-shot settings guard. That is 11a's seed-key finding stated as
// an API instead of as a warning: appending a rule to an existing group reaches
// fresh installs only, and a rule that must reach deployments already stamped
// takes a NEW group. The module names its groups, so the module makes that
// choice knowingly.
//
// Everything below is a shape check. Nothing here writes: `engagement/
// moduleSeeds.js` does, through the same `seedOne` and the same block validator
// core's own seeds go through.

// A module template key must be namespaced to its owner, for the same reason a
// trigger id must: `engagement_templates.key` is UNIQUE across the table, so an
// unprefixed `notify.event` from a module would collide with core's — and win or
// lose depending on boot order, which is the worst of both.
const TEMPLATE_KEY = /^[a-z][a-z0-9]*(?:[.-][a-z0-9]+)*$/
const MAX_TEMPLATE_KEY = 96
const SEED_GROUP_KEY = /^[a-z][a-z0-9]*(?:[-.][a-z0-9]+)*$/

// The channels a seeded template may target. Deliberately a literal rather than
// a read of the channel registry: this runs at registration time, which is
// before any channel a module might add is registered, and a seed for a channel
// nothing delivers is a row an operator can never use.
const SEEDABLE_CHANNELS = ['email', 'inapp']

// Core's own seed keys, which a module's rule MAY point at — that is §4.6.1
// property 1 in force, and the nine plain bodies of decision 9 are exactly this.
// Required lazily-safe: `templateSeeds` is pure data with no requires of its own.
// eslint-disable-next-line global-require
const coreTemplateKeys = () => new Set(require('../engagement/templateSeeds').SEEDS.map((s) => s.key))

function checkSeedTemplate(owner, entry) {
  const t = entry || {}
  const where = `registerEngagementSeeds: template "${t.key}"`
  if (!TEMPLATE_KEY.test(t.key || '') || t.key.length > MAX_TEMPLATE_KEY) {
    throw new Error(`registerEngagementSeeds: bad template key "${t.key}"`)
  }
  if (!t.key.startsWith(`${owner}.`)) {
    throw new Error(`${where} is not namespaced "${owner}."`)
  }
  if (!t.name) throw new Error(`${where} has no name`)
  if (!SEEDABLE_CHANNELS.includes(t.channel)) {
    throw new Error(`${where} has unknown channel "${t.channel}" (one of ${SEEDABLE_CHANNELS.join(', ')})`)
  }
  if (!Array.isArray(t.blocks) || !t.blocks.length) throw new Error(`${where} has no blocks`)
  if (!Number.isInteger(t.seedVersion) || t.seedVersion < 1) {
    throw new Error(`${where} needs an integer seedVersion of 1 or more`)
  }
  // An email body without a subject is a mail with an empty subject line, which
  // no operator meant; an in-app body WITH one is a column the inbox does not
  // read (`inapp.event` leaves it NULL and says why).
  if (t.channel === 'email' && !t.subject) throw new Error(`${where} is an email body with no subject`)
  if (t.channel !== 'email' && t.subject) {
    throw new Error(`${where} is a ${t.channel} body and cannot carry a subject`)
  }
  // `protected` is core's alone. It means "the system breaks without this body",
  // which is true of a password reset and true of nothing a module ships; a
  // module marking its own template undeletable is a module taking an operator's
  // delete button away.
  if (t.protected) throw new Error(`${where} may not be protected — that flag is core's`)
  return {
    key: t.key,
    name: t.name,
    channel: t.channel,
    subject: t.subject || null,
    blocks: t.blocks,
    seedVersion: t.seedVersion,
    triggerId: t.triggerId || null,
    triggerVersion: Number.isInteger(t.triggerVersion) ? t.triggerVersion : null,
    protected: false,
    status: 'published',
  }
}

function checkSeedRule(owner, entry, ownTemplateKeys, coreKeys) {
  const r = entry || {}
  const where = `registerEngagementSeeds: rule for "${r.trigger_id}"`
  if (!EVENT_ID.test(r.trigger_id || '')) {
    throw new Error(`registerEngagementSeeds: bad rule trigger_id "${r.trigger_id}"`)
  }
  // A module seeds rules for ITS OWN triggers. Shipping one for core's — or for
  // another module's — would mean uninstalling this module leaves a rule behind
  // that nobody can explain, and two modules could ship two rules for the same
  // event with neither aware of the other.
  if (!namespaced(owner, r.trigger_id, LEGACY_STREAM_IDS)) {
    throw new Error(`${where} is not namespaced "${owner}."`)
  }
  if (!r.name) throw new Error(`${where} has no name`)
  if (!Array.isArray(r.channels) || !r.channels.length) throw new Error(`${where} has no channels`)
  if (!r.audience) throw new Error(`${where} has no audience`)
  if (!Number.isInteger(r.cooldown_seconds) || r.cooldown_seconds < 0) {
    throw new Error(`${where} needs a cooldown_seconds of 0 or more`)
  }
  // Q3's hard ceiling, and the reason a seeded rule cannot omit it: it is what
  // keeps a misconfiguration from becoming a mail storm, so a module may choose
  // the number and may not decline to have one.
  if (!Number.isInteger(r.max_sends_per_hour) || r.max_sends_per_hour < 1) {
    throw new Error(`${where} needs a max_sends_per_hour of 1 or more`)
  }
  const keys = r.template_keys || {}
  if (!keys || typeof keys !== 'object' || Array.isArray(keys)) {
    throw new Error(`${where} needs a template_keys object`)
  }
  for (const [channel, key] of Object.entries(keys)) {
    // `digest` is a template slot rather than a channel — the digest worker's
    // body for a rule whose email channel is set to digest mode — so it is
    // allowed here and absent from `channels`.
    if (!ownTemplateKeys.has(key) && !coreKeys.has(key)) {
      throw new Error(
        `${where} names template "${key}" for ${channel}, which is neither one of its own seeds nor core's`,
      )
    }
  }
  return {
    trigger_id: r.trigger_id,
    name: r.name,
    audience: r.audience,
    audience_segment_id: null,
    channels: [...r.channels],
    template_keys: { ...keys },
    conditions: r.conditions === undefined ? null : r.conditions,
    cooldown_seconds: r.cooldown_seconds,
    delay_seconds: Number.isInteger(r.delay_seconds) ? r.delay_seconds : 0,
    cancel_on: Array.isArray(r.cancel_on) ? [...r.cancel_on] : [],
    // Never negotiable and never a parameter (Q3). A module that could ship an
    // enabled rule could mail a deployment's whole user table on the strength of
    // an upgrade nobody read the release note for.
    enabled: 0,
    updated_by: null,
  }
}

/**
 * `registerEngagementSeeds({ templates, ruleGroups })`.
 *
 * Validated whole, exactly as `apply()` validates: a module that got one of
 * thirty-two templates wrong ships none of them, and finds out at boot with the
 * offending key named rather than at send time with a half-seeded table.
 */
function checkEngagementSeeds(owner, entry) {
  const e = entry || {}
  if (e.templates !== undefined && !Array.isArray(e.templates)) {
    throw new Error('registerEngagementSeeds: templates must be an array')
  }
  if (e.ruleGroups !== undefined && !Array.isArray(e.ruleGroups)) {
    throw new Error('registerEngagementSeeds: ruleGroups must be an array')
  }
  const templates = []
  const seenKeys = new Set()
  for (const t of e.templates || []) {
    const checked = checkSeedTemplate(owner, t)
    if (seenKeys.has(checked.key)) {
      throw new Error(`registerEngagementSeeds: template "${checked.key}" declared twice`)
    }
    seenKeys.add(checked.key)
    templates.push(checked)
  }

  const coreKeys = coreTemplateKeys()
  const ruleGroups = []
  const seenGroups = new Set()
  for (const g of e.ruleGroups || []) {
    const group = g || {}
    if (!SEED_GROUP_KEY.test(group.key || '')) {
      throw new Error(`registerEngagementSeeds: bad rule group key "${group.key}"`)
    }
    if (seenGroups.has(group.key)) {
      throw new Error(`registerEngagementSeeds: rule group "${group.key}" declared twice`)
    }
    seenGroups.add(group.key)
    if (!Array.isArray(group.rules) || !group.rules.length) {
      throw new Error(`registerEngagementSeeds: rule group "${group.key}" has no rules`)
    }
    ruleGroups.push({
      key: group.key,
      note: group.note || '',
      rules: group.rules.map((r) => checkSeedRule(owner, r, seenKeys, coreKeys)),
    })
  }
  return { templates, ruleGroups }
}

/** Every registrant's seeds, in registration order. What the seeder walks. */
const allEngagementSeeds = () =>
  [...engagementSeeds.entries()].map(([owner, seeds]) => ({ owner, ...seeds }))

/** One registrant's, or null. */
const engagementSeedsFor = (owner) => engagementSeeds.get(owner) || null

// `specFile` is CORE-ONLY and is not on the module-facing signature. A slot's
// router reaches the app through declareSlot(), which no static parse of app.js
// can follow, so swagger-autogen would silently drop every route in it — the
// spike's exact failure (MODULE_API.md §7.4). Core names the file so
// `npm run swagger` can generate a fragment from it and merge it into the
// committed spec. A MODULE has no equivalent need: it ships a prebuilt
// `swagger-fragment.json` in its bundle (§6.1a), because core never has its
// sources to analyse.
function checkExtensionShape(slot, router, specFile) {
  if (!slots.has(slot)) throw new Error(`unknown extension slot "${slot}"`)
  if (typeof router !== 'function') throw new Error(`registerExtension: ${slot} is not a router`)
  return { slot, router, specFile: specFile || null }
}

// ── Staging + commit ───────────────────────────────────────────────────────

/**
 * A registrant's staging area: shape-checked claims, not yet visible to anyone.
 *
 * The loader hands one of these to a module through `api`, and `registerCore()`
 * builds one for core. Nothing a registrant says is readable through
 * `allStreams()` / `announceLeg()` / the slot routers until `apply()`.
 */
function stage(owner) {
  const staged = {
    owner,
    streams: [],
    legs: [],
    extensions: [],
    postHooks: [],
    teamProviders: [],
    slashCommands: [],
    triggers: [],
    audiences: [],
    engagementSeeds: [],
  }
  return {
    staged,
    registerNotificationStreams(entries) {
      if (!Array.isArray(entries)) throw new Error('registerNotificationStreams: expected an array')
      for (const e of entries) staged.streams.push(checkStreamShape(e))
    },
    registerAnnounceLeg(entry) {
      staged.legs.push(checkLegShape(entry))
    },
    registerExtension(slot, router, specFile) {
      staged.extensions.push(checkExtensionShape(slot, router, specFile))
    },
    registerPostHook(entry) {
      staged.postHooks.push(checkPostHookShape(entry))
    },
    registerTeamProvider(entry) {
      staged.teamProviders.push(checkTeamProviderShape(entry))
    },
    registerSlashCommands(entries) {
      if (!Array.isArray(entries)) throw new Error('registerSlashCommands: expected an array')
      for (const e of entries) staged.slashCommands.push(checkSlashCommandShape(e))
    },
    registerEventTriggers(entries) {
      if (!Array.isArray(entries)) throw new Error('registerEventTriggers: expected an array')
      for (const e of entries) staged.triggers.push(checkTriggerShape(e))
    },
    registerAudiences(entries) {
      if (!Array.isArray(entries)) throw new Error('registerAudiences: expected an array')
      for (const e of entries) staged.audiences.push(checkAudienceShape(e))
    },
    registerEngagementSeeds(entry) {
      staged.engagementSeeds.push(checkEngagementSeeds(owner, entry))
    },
  }
}

/**
 * Validate a staged batch against everything already registered, then commit it.
 *
 * Validation is TOTAL before the first write, so this either takes all of a
 * registrant's claims or none of them. Throws on the first collision, naming who
 * holds the thing already — which is the message an operator needs and the one
 * PR 2 learned to protect (mounting inside the scan loop made every collision
 * look like it was with core).
 */
function apply({
  owner,
  streams: newStreams,
  legs: newLegs,
  extensions: newExtensions,
  postHooks: newPostHooks = [],
  teamProviders: newTeamProviders = [],
  slashCommands: newSlashCommands = [],
  triggers: newTriggers = [],
  audiences: newAudiences = [],
  engagementSeeds: newSeeds = [],
}) {
  // ── validate ──
  const seenStreams = new Set()
  for (const s of newStreams) {
    const held = streamOwners.get(s.id)
    if (held) throw new Error(`stream "${s.id}" is already registered by "${held}"`)
    if (seenStreams.has(s.id)) throw new Error(`stream "${s.id}" registered twice`)
    // The cross-facet half of the one-namespace rule (§7.2). A stream may share
    // its id with a TRIGGER — that is the whole point, `news.post` is one event
    // with two facets — but only when the same registrant owns both. Someone
    // else's trigger id is taken.
    const heldAsTrigger = triggers.get(s.id)
    if (heldAsTrigger && heldAsTrigger.owner !== owner) {
      throw new Error(`stream "${s.id}" is already registered as an event trigger by "${heldAsTrigger.owner}"`)
    }
    if (!namespaced(owner, s.id, LEGACY_STREAM_IDS)) {
      throw new Error(`stream "${s.id}" is not namespaced "${owner}."`)
    }
    seenStreams.add(s.id)
  }

  // Triggers, against the SAME namespace and the SAME legacy allowlist as
  // streams above. Sharing LEGACY_STREAM_IDS is not laziness: under one
  // namespace `idoc.warning` is one id, so if `uo` may hold it as a stream
  // without the prefix it may hold it as a trigger without the prefix, and any
  // other answer would mean the seven grandfathered ids could never gain a
  // payload contract.
  const seenTriggers = new Set()
  for (const t of newTriggers) {
    const held = triggers.get(t.id)
    if (held) throw new Error(`event trigger "${t.id}" is already registered by "${held.owner}"`)
    if (seenTriggers.has(t.id)) throw new Error(`event trigger "${t.id}" registered twice`)
    const heldAsStream = streamOwners.get(t.id)
    if (heldAsStream && heldAsStream !== owner) {
      throw new Error(`event trigger "${t.id}" is already registered as a notification stream by "${heldAsStream}"`)
    }
    if (!namespaced(owner, t.id, LEGACY_STREAM_IDS)) {
      throw new Error(`event trigger "${t.id}" is not namespaced "${owner}."`)
    }
    seenTriggers.add(t.id)
  }

  const seenAudiences = new Set()
  for (const a of newAudiences) {
    const held = audiences.get(a.id)
    if (held) throw new Error(`audience "${a.id}" is already registered by "${held.owner}"`)
    if (seenAudiences.has(a.id)) throw new Error(`audience "${a.id}" registered twice`)
    // No legacy allowlist — nothing predates audiences, so the prefix rule has no
    // exceptions and should never grow one.
    if (!namespaced(owner, a.id, {})) {
      throw new Error(`audience "${a.id}" is not namespaced "${owner}."`)
    }
    seenAudiences.add(a.id)
  }

  const seenLegs = new Set()
  for (const l of newLegs) {
    const held = legs.get(l.leg)
    if (held) throw new Error(`announce leg "${l.leg}" is already registered by "${held.owner}"`)
    if (seenLegs.has(l.leg)) throw new Error(`announce leg "${l.leg}" registered twice`)
    if (!namespaced(owner, l.leg, LEGACY_LEGS)) {
      throw new Error(`announce leg "${l.leg}" is not namespaced "${owner}."`)
    }
    seenLegs.add(l.leg)
  }

  const seenSlots = new Set()
  for (const x of newExtensions) {
    const entry = slots.get(x.slot)
    if (entry.filledBy) {
      throw new Error(`extension slot "${x.slot}" is already filled by "${entry.filledBy}"`)
    }
    if (seenSlots.has(x.slot)) throw new Error(`extension slot "${x.slot}" filled twice`)
    seenSlots.add(x.slot)
  }

  // One call per registrant, like the post hook and the team provider above it.
  // A second call is a module that wrote its seeds in two places, and merging
  // them silently would make "which group is this rule in" unanswerable.
  if (newSeeds.length > 1) throw new Error(`"${owner}" registered engagement seeds more than once`)
  if (newSeeds.length && engagementSeeds.has(owner)) {
    throw new Error(`"${owner}" already registered engagement seeds`)
  }

  if (newPostHooks.length > 1) throw new Error(`"${owner}" registered more than one post hook`)
  if (newPostHooks.length && postHooks.has(owner)) {
    throw new Error(`"${owner}" already registered a post hook`)
  }

  if (newTeamProviders.length > 1) throw new Error(`"${owner}" registered more than one team provider`)
  if (newTeamProviders.length && teamProvider) {
    throw new Error(`a team provider is already registered by "${teamProvider.owner}"`)
  }

  const seenCommands = new Set()
  for (const c of newSlashCommands) {
    const held = slashCommands.get(c.name)
    if (held) throw new Error(`slash command "/${c.name}" is already registered by "${held.owner}"`)
    if (seenCommands.has(c.name)) throw new Error(`slash command "/${c.name}" registered twice`)
    seenCommands.add(c.name)
  }

  // ── commit — nothing below can fail ──
  for (const s of newStreams) {
    streamOwners.set(s.id, owner)
    streams.push(s)
  }
  for (const l of newLegs) legs.set(l.leg, { owner, ...l })
  for (const x of newExtensions) {
    const entry = slots.get(x.slot)
    entry.filledBy = owner
    entry.specFile = x.specFile
    entry.router.use(x.router)
  }
  for (const h of newPostHooks) postHooks.set(owner, h)
  for (const p of newTeamProviders) teamProvider = { owner, ...p }
  for (const c of newSlashCommands) slashCommands.set(c.name, { owner, ...c })
  for (const t of newTriggers) triggers.set(t.id, { owner, ...t })
  for (const a of newAudiences) audiences.set(a.id, { owner, ...a })
  for (const seeds of newSeeds) engagementSeeds.set(owner, seeds)
}

// ── Core's own registrations ───────────────────────────────────────────────

/**
 * Register everything CORE owns, through the same staging area a module uses.
 *
 * Called once from app.js, before `modules.load()` — before, because a module's
 * collision checks are asked against what is already registered, and core's
 * claims must be the ones already there.
 *
 * What is here is what survives Phase 3. Everything after the boundary comment is
 * shard content and leaves with module-uo, registered rather than hardcoded so
 * the seam is exercised on every boot long before a module first uses it.
 */
function registerCore() {
  if (coreRegistered) return

  /* eslint-disable global-require */
  const coreStreams = require('../config/coreStreams')
  const coreTriggers = require('../config/coreTriggers')
  const discordLeg = require('../utils/discordAnnounce')
  /* eslint-enable global-require */

  const api = stage('core')
  api.registerNotificationStreams(coreStreams.STREAMS)
  api.registerAnnounceLeg(discordLeg.leg)
  // The engagement contract (ENGAGEMENT.md Phase 2). Core's five trigger ids ARE
  // its five stream ids — the same-owner upgrade the one-namespace rule above is
  // written for — so this batch exercises the cross-facet check on every boot.
  api.registerEventTriggers(coreTriggers.TRIGGERS)

  // The three lines that used to follow — the shard stream catalog, the town
  // crier leg and the `admin.users.detail` filling — were shard CONTENT held
  // here so the seam would be exercised on every boot before a module first used
  // it. Phase 3 moved them into module-uo's `register()` verbatim, with 'core'
  // becoming 'uo', and nothing else in core changed. That was the claim PR 4
  // made, and this deletion is it being collected.
  apply(api.staged)
  coreRegistered = true

  log.info('core registrations complete', {
    streams: streams.length,
    eventTriggers: triggers.size,
    announceLegs: legs.size,
    extensions: [...slots.keys()].filter(slotFilledBy),
  })
}

/** Has registerCore() run? Read by tests, and by the loader's ordering assertion. */
const isCoreRegistered = () => coreRegistered

// Test-only: hand the process back. Registries are process-global by design
// (there is one core), so a test that registers has to be able to undo it.
//
// Slot DECLARATIONS survive, and only their fills are cleared: a slot is declared
// at require time by the router that owns the resource, and that require has
// already happened and will not happen again in this process. Clearing the map
// would leave a slot that nothing can re-declare. The cost is that a test filling
// the same slot twice stacks two routers inside it; no test reads through a slot
// router, so that is left rather than papered over with a rebuilt router that
// would no longer be the object users.router.js mounted.
function _reset() {
  for (const entry of slots.values()) {
    entry.filledBy = null
    entry.specFile = null
  }
  streams.length = 0
  streamOwners.clear()
  legs.clear()
  postHooks.clear()
  teamProvider = null
  slashCommands.clear()
  triggers.clear()
  audiences.clear()
  engagementSeeds.clear()
  coreRegistered = false
}

module.exports = {
  declareSlot,
  hasSlot,
  slotFilledBy,
  filledSlots,
  declaredSlotRouter,
  allStreams,
  isValidStream,
  personalStreams,
  announceLegs,
  announceLegIds,
  announceLeg,
  postHookEntries,
  dispatchPostHook,
  registeredTeamProvider,
  hasTeamProvider,
  slashCommandDefinitions,
  slashCommand,
  allTriggers,
  eventTrigger,
  eventOwner,
  allAudiences,
  audience,
  resolveAudience,
  allEngagementSeeds,
  engagementSeedsFor,
  SEEDABLE_CHANNELS,
  VARIABLE_TYPES,
  TRIGGER_KINDS,
  stage,
  apply,
  registerCore,
  isCoreRegistered,
  _reset,
}