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/gen_fixtures.rs

@@ -37,6 +37,7 @@ fn fixture_snapshot(tick: u64, entities: Vec<VisibleEntity>) -> ObserverSnapshot
         dialogue_response: None,
         blocked_entities: vec![],
         scan_events: vec![],
+        rng_seed: None,
     }
 }
 
@@ -208,6 +209,7 @@ fn generate_msgpack_fixtures() {
         dialogue_response: None,
         blocked_entities: vec![],
         scan_events: vec![],
+        rng_seed: None,
     };
     write_fixture(
         "snapshot_v2_full",