feat(modules): the filesystem module loader

Phase 2 PR 2 of docs/website/MODULE_SYSTEM.md 2.7. Adds
server/src/modules/{loader,semver,version}.js: the synchronous scan of
MODULES_DIR, manifest validation, prefix and table-name collision
rejection, per-module try/catch and the mount into the three tier
routers behind the MODULE_API.md 4.5 dispatch guard.

Two decisions the contract left open, both now written up there:

- The load trigger is one explicit modules.load(tierRouters) call in
  app.js, not a lazy scan (API 7.6). Accessors throw until it has run,
  because "no modules installed" is a real answer a caller must not be
  handed by accident.
- Whether core owns a prefix is asked of the live tier routers via
  express's own layer.match(), skipping root-mounted layers, rather than
  a hardcoded table -- the spike's was already stale when written
  (API 4.3).

Mounting is a second pass after every module is validated. Doing it
inside the scan loop makes the first module's layers indistinguishable
from core's, so the second module claiming a taken prefix is told it
collided with core and the module-versus-module check is unreachable.

registerExtension/NotificationStreams/AnnounceLeg and onBoot/onShutdown
throw "not available until phase 2 PR 4/5" rather than no-op; an
accepting stub would let a module believe it had registered something.
No schema replay, no boot dispatch, no installed_modules reconcile --
those are PRs 3 and 5, and until PR 5 a record's state is in memory only.

No module ships on the volume, so nothing an operator or client can see
changes: 842 tests pass, routes.manifest.json is unchanged at 229 routes
and swagger-output.json regenerates byte-identical.

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

server/src/modules/version.js

@@ -0,0 +1,14 @@
+// The module API version — the single number a module's `coreApi` range is
+// checked against (docs/website/MODULE_API.md §1.1).
+//
+// Bump minor when a member is ADDED to ctx or a new register* call appears;
+// major when one is removed, its signature changes, or its behaviour changes
+// without a signature change. A core-internal refactor behind an unchanged
+// member is not a bump.
+//
+// Deliberately separate from PROTOCOL_VERSION (which versions the shard wire and
+// has nothing to say about a website module) and from any module's own version.
+
+const MODULE_API_VERSION = '1.0.0'
+
+module.exports = { MODULE_API_VERSION }