feat(modules): the client registry, window.__rg and the chunk's script injection

Phase 2, PR 7 of docs/website/MODULE_SYSTEM.md 2.7 — the client half's
delivery. A module's prebuilt chunk is served, injected, handed core's React
and its UI kit, and its routes are rendered by App.jsx. The registry is empty
on a bare core, so nothing an operator can see changes.

Client:
  - modules/registry.js — registerRoutes/registerNav/registerFeatureProvider,
    with the URL namespace written by core, never by the module
  - modules/shared.js — window.__rg: React, react-dom/client, react-router-dom,
    react/jsx-runtime, the registry, the seven-member UI kit and the request
    primitive, frozen
  - App.jsx reads routesFor for all three areas; nav consumption is PR 8
  - main.jsx publishes the global, then mounts on DOMContentLoaded

Server:
  - the loader validates client.entry and publishes clientChunks() and
    clientEntryUrls(); an entry in the module root is rejected, because the
    directory it sits in is what gets served
  - app.js mounts each chunk at /modules/<id>/ behind the module's state guard
    with no-cache; anything else under /modules is a 404, not the SPA shell
  - htmlShell injects the tag before </body>, so core's bundle runs first
    wherever a bundler puts it

Found by loading a real chunk in a browser, and fixed here: core mounted before
any module chunk had evaluated, because document.readyState during a deferred
script is 'interactive', not 'loading'. Every test passed against that build.
The smoke is written down in MODULE_API.md 7.7.

933 server tests (+23), 123 client tests (+14). routes.manifest.json unchanged
at 230 routes; the OpenAPI spec regenerates byte-identical.

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

server/src/app.js

@@ -186,6 +186,47 @@ modules.load({
 
 app.use('/api', (req, res) => res.status(404).json({ message: 'Not found' }))
 
+// Installed modules' prebuilt client chunks, at /modules/<id>/ — same-origin, so
+// `script-src 'self'` admits them with no nonce and no inline script
+// (docs/website/MODULE_API.md §3.1). Three properties, each load-bearing:
+//
+//   • The static root is the directory the ENTRY sits in, never the module root.
+//     One express.static over a module root would publish its server source, its
+//     module.json and its schema fragment; the loader rejects an entry that would
+//     make those the same directory.
+//   • Behind the module's own state guard, so a failed module's chunk is 503 and
+//     a disabled one's is 404 — the same answers its API gives, for the same
+//     reason: the browser should not be running the client half of something the
+//     server half has stopped serving.
+//   • `fallthrough: false`, so a missing file is a 404 here rather than falling
+//     through to the SPA catch-all and answering a `<script src>` with the index
+//     shell, which the browser then rejects on its MIME type instead.
+//
+// Vite's library build emits an unhashed `entry.js`, so `no-cache` (revalidate,
+// not "do not store") is what stops an upgraded module serving yesterday's chunk
+// out of the disk cache.
+for (const chunk of modules.clientChunks()) {
+  app.use(
+    chunk.url,
+    chunk.guard,
+    express.static(chunk.dir, {
+      fallthrough: false,
+      setHeaders: (res) => {
+        res.set('Cache-Control', 'no-cache')
+        res.set('X-Content-Type-Options', 'nosniff')
+      },
+    }),
+  )
+}
+
+// Everything else under /modules is a 404, not the SPA shell. The namespace
+// belongs to installed modules' chunks — an unknown module id or a file a module
+// does not ship is a missing file, and answering a `<script src>` with an HTML
+// page turns that into a MIME-type refusal in the console with a 200 in the
+// network tab. It also keeps the namespace's boundary a fact of the app rather
+// than of whichever catch-all happens to be mounted after it.
+app.use('/modules', (req, res) => res.status(404).json({ message: 'Not found' }))
+
 // ── /.well-known ──────────────────────────────────────────────────────
 // Android App Links verification file at the web root (M9 follow-up). Mounted
 // before the SPA catch-all so it returns JSON, not the index shell. 404s unless