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

Add rng_seed: Option<u64> to ObserverSnapshot (protocol v10). Populated
from SimRng state each tick. Completes the WRONG button capture loop —
seed.txt now writes a valid u64 instead of "unavailable", enabling
deterministic replay from bug reports.

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

server/src/bridge/text_renderer.rs

@@ -299,6 +299,7 @@ mod tests {
             dialogue_response: None,
             blocked_entities: vec![],
             scan_events: vec![],
+            rng_seed: None,
         }
     }
 
@@ -422,6 +423,7 @@ mod tests {
             dialogue_response: None,
             blocked_entities: vec![],
             scan_events: vec![],
+            rng_seed: None,
         };
         let text = format_snapshot_text(&snap);
         assert!(text.contains("Tick 0"));