feat(13-07): music.* RPC surface behind one dispatcher arm

- api/rpc/music.rs: handle_music prefix sub-dispatcher (assistant_chat.rs
  shape) with list-albums / list-artists / list-tracks / status / reindex
- one guarded dispatcher.rs arm for the whole music. prefix, adjacent to
  the content.* block; the only registration point for the surface
- list-tracks: optional album_id filter, limit/offset pagination, limit
  clamped to [1,500] (default 100) — out-of-range degrades, never errors
  (T-13-41)
- reindex spawns the scan and returns immediately; second call while one
  runs reports already-running with the last stats; optional
  incremental:true routes to refresh_incremental so a changed library is
  reflected without a full re-extraction
- newer-schema index served as an empty library, never overwritten or
  reinterpreted by readers (13-MUSIC-MODEL.md downgrade contract)
- nothing music.* in UNAUTHENTICATED_METHODS — the surface rides the
  session/CSRF/RBAC gate; asserted by music_methods_require_session
  (T-13-40)
- 7 tests, one per Task 2 behavior bullet plus the incremental mode

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

core/archipelago/src/api/rpc/mod.rs

@@ -22,6 +22,7 @@ mod marketplace;
 mod mesh;
 mod middleware;
 mod monitoring;
+mod music;
 mod names;
 mod network;
 mod node;