feat: declare rust as the module's identity capability

Phase 5 is the Android app's leg of this module's read path (R10), and it
gates its Rust navigation on one capability string the way `module-uo`'s five
shard rows gate on `shard`. There was no such string here: the five this module
declared all name a SURFACE, and core flattens every started module's
capabilities into one list, so `servers` is a word another module could declare
tomorrow and silently reveal these screens on a site that does not run Rust.

`rust` is the string only this module can mean. It is asserted against
`module.json`'s own `id` rather than a literal, so the two cannot drift.

The README says why it is not redundant with `id`: `id` is a mount prefix, and
MODULE_API.md §2.9 forbids a client inferring a route from a capability. Gating
on `id` would quietly make those the same thing.

Decided by the org lead as D16, 2026-09-16.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016wDDVXWMDz82WqE1i969r4

server/test/entry.test.js

@@ -148,3 +148,20 @@ test('the module’s protocol version agrees with the manifest it ships beside',
   assert.strictEqual(typeof sidecar.PROTOCOL_VERSION, 'number')
   assert.ok(sidecar.PROTOCOL_VERSION >= 1)
 })
+
+test('an identity capability is declared, and it is the module id (phase 5, D16)', () => {
+  // Core flattens every started module's capabilities into ONE list, so a client
+  // asking "is this module installed" needs a string only this module can
+  // declare. `servers` is not that string — it names a surface, and another
+  // module could name it too — which is the whole reason this one exists beside
+  // the five surface words.
+  //
+  // It is asserted against `manifest.id` rather than against the literal "rust"
+  // so that the two cannot drift: the day the id changes, the capability a
+  // client gates a whole navigation group on has to change with it.
+  assert.ok(
+    manifest.capabilities.includes(manifest.id),
+    `module.json must declare "${manifest.id}" as a capability — it is the only string a client can` +
+      ' use to tell this module apart from any other, and the Android app gates its Rust rows on it',
+  )
+})