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/tests/serialization.rs

@@ -7,7 +7,7 @@ use std::fs;
 /// Helper to create a minimal v2 snapshot for tests
 fn test_snapshot(tick: u64, entities: Vec<VisibleEntity>) -> ObserverSnapshot {
     ObserverSnapshot {
-        version: 2,
+        version: 3,
         tick,
         game_time: GameTime {
             day: 0,
@@ -40,7 +40,7 @@ fn observer_snapshot_roundtrip() {
     let bytes = rmp_serde::to_vec_named(&snapshot).expect("serialize");
     let decoded: ObserverSnapshot = rmp_serde::from_slice(&bytes).expect("deserialize");
 
-    assert_eq!(decoded.version, 2);
+    assert_eq!(decoded.version, 3);
     assert_eq!(decoded.tick, 42);
     assert_eq!(decoded.entities.len(), 1);
     assert_eq!(decoded.entities[0].entity_id, 1);
@@ -178,7 +178,7 @@ fn all_entity_kind_variants_roundtrip() {
 #[test]
 fn snapshot_v2_fields_roundtrip() {
     let snapshot = ObserverSnapshot {
-        version: 2,
+        version: 3,
         tick: 100,
         game_time: GameTime {
             day: 3,
@@ -216,7 +216,7 @@ fn snapshot_v2_fields_roundtrip() {
     let bytes = rmp_serde::to_vec_named(&snapshot).expect("serialize");
     let decoded: ObserverSnapshot = rmp_serde::from_slice(&bytes).expect("deserialize");
 
-    assert_eq!(decoded.version, 2);
+    assert_eq!(decoded.version, 3);
     assert_eq!(decoded.game_time.day, 3);
     assert_eq!(decoded.game_time.time_of_day, 720);
     assert_eq!(decoded.game_time.day_phase, DayPhase::Evening);
@@ -244,7 +244,7 @@ fn all_facing_direction_variants_roundtrip() {
 
     for dir in directions {
         let snapshot = ObserverSnapshot {
-            version: 2,
+            version: 3,
             tick: 0,
             game_time: GameTime {
                 day: 0,