feat(modules): interleave module nav, derive moderator confinement

Phase 2, PR 8 of docs/website/MODULE_SYSTEM.md 2.7 - the nav half PR 7
deferred, plus the two seams 1.4 and 1.5 asked for.

withModuleNav (client/src/modules/nav.js) merges an installed module's rows
into core's three navs BEFORE the admin-override merge, and that ordering is
the design. applyNavOverrides and buildPublicNav are keyed by `to` and drop
any key their base array does not declare, so rows appended after the merge
would be unorderable, unrelabellable and unhideable in Admin - Navigation.
Today's UO rows are all three of those things, so appending would make the
extraction a visible regression for anyone who has ever edited their nav.
Merging first means a module row is an ordinary row downstream: nothing in
navOverrides.js, NavEditor.jsx or the layouts knows a module exists.

MOD_PATHS is gone. Moderator visibility and the redirect that confines a
moderator both derive from each row's own `roles`, in the new plain-JS
lib/adminNav.js (plain so the DOM-less runner can reach it). Two rows move,
both toward what the server already permitted: Dashboard, whose roles had
always named moderator, and My Characters, which is ungated self-service.

That also fixes a defect predating the module system. The redirect was a
THIRD hardcoded list - three path prefixes against MOD_PATHS' five paths -
and they disagreed about /admin/houses, so a moderator who clicked Houses in
their own sidebar was bounced back to Moderation. The derived allow-list is
computed from the BASE nav, never the override-merged one: an override is
presentation and must not move an authorization boundary either way.

The feature seam (modules/features.jsx + modules/featureGate.js) resolves a
row's `feature` against the provider its OWN module registered, so the
namespace comes from the registration and no string carries a parsed prefix.
Core registers useShardFlags under the owner id `core` - the client twin of
registries.registerCore() - so the ten shard-gated header rows already run
through the seam and Phase 3 deletes a registration instead of rewriting
SiteHeader. Every unknown fails open: no provider, a null answer while a
fetch is in flight, or a junk return all show the link, because the server is
the gate and hiding a page from someone entitled to it is the worse mistake.

933 server tests (unchanged - this PR is client-only), 160 client tests
(+37). routes.manifest.json unchanged at 230 routes; the OpenAPI spec
regenerates byte-identical.

Re-ran the MODULE_API.md 7.7 browser smoke, since this is the seam that rule
exists for. A throwaway module registering nav in all three areas and a
provider granting one flag and withholding another: the row lands inside
core's Moderation group rather than an appended block, the withheld row does
not render, a moderator reaches both /admin/houses and the module's admin
page, and an admin can relabel a module row and have it persist and apply.
Zero CSP reports, zero console errors.

Co-Authored-By: Claude <noreply@anthropic.com>

client/test/moduleRegistry.test.js

@@ -12,6 +12,7 @@ import {
   routesFor,
   navFor,
   featureProviderFor,
+  featureProviders,
   registeredIds,
   _reset,
 } from '../src/modules/registry.js'
@@ -126,6 +127,28 @@ test('a feature provider is stored under its namespace, with its owner', () => {
   assert.equal(featureProviderFor('nothing'), undefined)
 })
 
+test('providers can be enumerated in registration order, with their owner', () => {
+  // Core's feature context has to CALL each of these, as a hook, in a fixed
+  // order — so it needs the list, and it needs the owner id to resolve a nav
+  // row whose `moduleId` says who it belongs to (modules/features.jsx).
+  const uo = () => null
+  const rust = () => null
+  registerFeatureProvider('uo', 'shard', uo)
+  registerFeatureProvider('rust', 'server', rust)
+  assert.deepEqual(featureProviders(), [
+    { id: 'uo', namespace: 'shard', hook: uo },
+    { id: 'rust', namespace: 'server', hook: rust },
+  ])
+})
+
+test('enumerating providers is NOT part of the module-facing surface', () => {
+  // A module asks for a namespace it knows the name of; enumerating what
+  // everyone else registered is core's business, so `featureProviders` is a
+  // module export and not a member of window.__rg.registry.
+  assert.equal(registry.featureProviders, undefined)
+  assert.equal(typeof featureProviders, 'function')
+})
+
 test('every registration marks the module registered', () => {
   registerRoutes('a', { public: [{ path: 'x' }] })
   registerNav('b', { area: 'public', items: [] })