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>

client/src/modules/version.js

@@ -0,0 +1,14 @@
+// The client's copy of MODULE_API_VERSION. It must equal the server's
+// (server/src/modules/version.js) — the two halves version ONE contract
+// (docs/website/MODULE_API.md §1.1), and a module checks whichever half it is
+// talking to: `coreApi` against the server's at load time, `window.__rg.version`
+// against the client's before it registers anything.
+//
+// Duplicated rather than fetched, and that is deliberate. The value has to be on
+// `window.__rg` before the first module chunk evaluates, which is earlier than
+// any network round trip could answer — a fetched version would mean either an
+// await before render or a module reading `undefined`. The cost of the copy is
+// that the two files can drift, so a test asserts they agree
+// (client/test/moduleRegistry.test.js) rather than trusting a bump to remember
+// both.
+export const MODULE_API_VERSION = '1.0.0'