fix(simulation): address PR #12 review — z-level filter, visible tile dedup, version bump

- Filter remembered entities by z-level (Hoshe + Tyre warning)
- Skip remembered ghosts on currently visible tiles (Hoshe warning)
- Bump ObserverSnapshot version to 3 (Tyre suggestion)
- Add edge case tests: visible tile collision, different z-level,
  knowledge without position (Hoshe suggestion)
- Regenerate msgpack fixtures for v3

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

server/src/bridge/mod.rs

@@ -107,7 +107,7 @@ pub fn generate_snapshot(
         visible.len()
     );
     buffer.snapshot = Some(ObserverSnapshot {
-        version: 2,
+        version: 3,
         tick: time.tick,
         game_time,
         player_facing: FacingDirection::default(),