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

module.json

@@ -12,5 +12,5 @@
     "admin": ["/rust"],
     "player": ["/rust"]
   },
-  "capabilities": ["servers", "killfeed", "leaderboard", "presence", "wipes"]
+  "capabilities": ["rust", "servers", "killfeed", "leaderboard", "presence", "wipes"]
 }