feat(bridge): add rng_seed to ObserverSnapshot for deterministic replay (#527)

Adds rng_seed: Option<u64> to ObserverSnapshot. The WRONG button (#507) captures
inputs.jsonl and seed.txt for replay, but seed.txt was writing "unavailable"
because the server did not include the RNG seed in ObserverSnapshot.

Changes:
- bridge/types.rs: PROTOCOL_VERSION 9→10, rng_seed field with serde(default,
  skip_serializing_if = "Option::is_none") for backward compatibility
- perception/observer/mod.rs: inject Res<SimRng> into compute_observer_snapshot,
  populate rng_seed: Some(rng.seed()) each tick
- All test files: add rng_seed: None to ObserverSnapshot constructors
- tests/serialization.rs: bump protocol_version_constant assertion 9→10
- Regenerate msgpack fixtures and golden file for protocol v10

Completes the WRONG button capture loop: replays can now fully reproduce
observed bugs with the exact RNG seed from the capture.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/tests/golden/proof_room_tick_10.json

@@ -64,9 +64,10 @@
   "player_facing": "North",
   "player_inventory": [],
   "player_stance": "Sprint",
+  "rng_seed": 42,
   "scan_events": [],
   "tick": 8,
-  "version": 9,
+  "version": 10,
   "visible_tiles": [
     {
       "tile_kind": "Wall",