feat(modules): client extension slots (phase 3, slice 2)

The client twin of the server's declareSlot/registerExtension, and the same rule
in both halves: core declares a slot, only core declares one, and at most one
module fills it. Core renders <Slot name> and gets nothing back when the slot is
unfilled, so an instance with no module installed renders exactly what it
rendered before -- the same untouched-path guarantee withModuleNav makes.

A slot is named for a PLACE, never for a meaning. Core supplies the position and
the styling; the label, the target, the data and whether anything renders at all
are the module's. The moment core types a slot by its content it has re-acquired
the game semantics phase 3 exists to remove.

This is the one place the client registry is not fail-open. An unknown slot, a
non-component and a second fill all throw, matching checkExtensionShape
server-side, because a dropped nav row costs a link the viewer can reach another
way while a silently dropped extension is invisible to everyone including its
author. A throw is always a programming error and never a race: core declares in
its own bundle and every module chunk is a deferred script injected after it.

Reading stays fail-safe -- undeclared and unfilled both read null -- and a
filling component renders inside an error boundary. That asymmetry is where the
client differs from the server: a module route that throws costs the module's own
page, but an extension throws inside CORE's, and the whole reason core keeps
ownership of that page is that it stays usable.

Core decorates a slot through <Slot wrap>, not by asking whether it is filled.
The obvious alternative is right about the unfilled case and wrong about the
failed one -- the extension is filled, so the separator renders, and then the
component throws into the boundary and leaves the separator behind on its own.
wrap puts core's decoration inside the boundary where it shares the extension's
fate. Found in a browser, with the footer's separator, which is the only place
either could have been found.

MODULE_API_VERSION 1.1.0 -> 1.2.0, both halves: the two state ONE version.
Contract: docs/website/MODULE_API.md 3.7.

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

client/src/modules/registry.js

@@ -32,6 +32,8 @@
 const routes = { public: [], admin: [], player: [] }
 const nav = { public: [], admin: [], player: [] }
 const providers = new Map()
+// slot name → { Component, filledBy }.
+const slots = new Map()
 const registered = new Set()
 
 const AREAS = ['public', 'admin', 'player']
@@ -100,6 +102,70 @@ export function registerFeatureProvider(id, namespace, hook) {
   registered.add(id)
 }
 
+// ── Extension slots (§3.7) ─────────────────────────────────────────────────
+//
+// The client twin of the server's declareSlot/registerExtension, and the same
+// rule in both halves: core declares a slot, ONLY core declares one, and at most
+// one module fills it. Core renders `<Slot name>` (Slot.jsx) and gets nothing
+// back when the slot is unfilled — so an instance with no module installed
+// renders exactly what it renders today.
+//
+// A slot is named for a PLACE, never for a meaning. `site.footer.status` is a
+// position in the footer and the styling that goes with it; the label, the
+// target, the data and whether anything renders at all are the module's. The
+// moment core types a slot by its content it has re-acquired the game semantics
+// this whole extraction removes.
+
+/**
+ * @param {string} name  the slot id. Core-only — deliberately not on the
+ *   `registry` object handed to modules.
+ */
+export function declareSlot(name) {
+  if (slots.has(name)) throw new Error(`extension slot "${name}" already declared`)
+  slots.set(name, { Component: null, filledBy: null })
+}
+
+/**
+ * Fill a declared slot with a component.
+ *
+ * **This is the one place the client registry is not fail-open**, and the
+ * asymmetry is deliberate. A dropped nav row costs a link the viewer can reach
+ * another way; a silently dropped extension is invisible to everyone including
+ * its author. So an unknown slot, a non-component, and a second fill all throw —
+ * exactly as the server's checkExtensionShape does.
+ *
+ * A throw here is always a programming error and never a race, because
+ * declaration structurally precedes filling: core declares in main.jsx, inside
+ * its own bundle, and every module chunk is a deferred script injected after it
+ * (§3.1).
+ */
+export function registerExtension(id, slot, Component) {
+  const entry = slots.get(slot)
+  if (!entry) throw new Error(`registerExtension: unknown extension slot "${slot}"`)
+  if (typeof Component !== 'function') throw new Error(`registerExtension: ${slot} is not a component`)
+  if (entry.filledBy) throw new Error(`extension slot "${slot}" is already filled by "${entry.filledBy}"`)
+  entry.Component = Component
+  entry.filledBy = id
+  registered.add(id)
+}
+
+/**
+ * The filling component, or null.
+ *
+ * Read by Slot.jsx and nothing else — deliberately. There is no `hasExtension`
+ * for a core layout to branch on, because a layout that asks whether a slot is
+ * filled and then renders its own decoration alongside gets the *failed* case
+ * wrong: the extension is filled, so the decoration renders, and the component
+ * then throws into the boundary leaving the decoration behind on its own. Core
+ * decorates through `<Slot wrap>` instead, which puts the decoration inside the
+ * boundary where it shares the extension's fate. (Found in a browser, with the
+ * footer's separator.)
+ *
+ * Undeclared and unfilled both read null: reading is fail-safe, and only writing
+ * is strict.
+ */
+export const extensionFor = (slot) => (slots.get(slot) || {}).Component || null
+
 export const routesFor = (area) => routes[area] || []
 
 // Sorted by the `order` a module asked for. Array#sort is stable in every engine
@@ -131,6 +197,11 @@ export function _reset() {
     nav[area].length = 0
   }
   providers.clear()
+  // Declarations go too, unlike the server's, where a slot is declared once at
+  // require time by the router that owns it. Core declares its slots in
+  // main.jsx — the one file no test loads — so on this side there is nothing
+  // declared at import time for a surviving declaration to protect.
+  slots.clear()
   registered.clear()
 }
 
@@ -142,6 +213,7 @@ export const registry = {
   registerRoutes,
   registerNav,
   registerFeatureProvider,
+  registerExtension,
   routesFor,
   navFor,
   featureProviderFor,