test(server): add Entity::to_bits roundtrip and TickRate switch tests

- Entity::to_bits() roundtrip test guards against bevy version changes
  silently breaking wire IDs (Hoshe #1)
- PROTOCOL_VERSION constant used in test helpers instead of hardcoded 4
- TickRate switch mid-accumulation test verifies Half→Full→Paused→Half
  transitions preserve accumulator state correctly (Tyre N3)
- POI mid-range test now asserts priority=1 (Hoshe #4)

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: 4,
+        version: PROTOCOL_VERSION,
         tick,
         game_time: GameTime {
             day: 0,
@@ -231,6 +231,35 @@ fn snapshot_v2_fields_roundtrip() {
     assert_eq!(decoded.entities[0].visibility, VisibilitySector::Forward);
 }
 
+/// Entity::to_bits() must roundtrip through from_bits() — guards against
+/// bevy version changes silently breaking wire IDs (Hoshe #12).
+#[test]
+fn entity_to_bits_roundtrip() {
+    use bevy_ecs::entity::Entity;
+    // Create entities via a World so we get valid index+generation pairs
+    let mut world = bevy_ecs::world::World::new();
+    let e1 = world.spawn_empty().id();
+    let e2 = world.spawn_empty().id();
+    let e3 = world.spawn_empty().id();
+    // Despawn and respawn to get a higher generation
+    world.despawn(e2);
+    let e4 = world.spawn_empty().id();
+
+    for entity in [e1, e2, e3, e4] {
+        let bits = entity.to_bits();
+        let restored = Entity::from_bits(bits);
+        assert_eq!(entity, restored, "Entity::to_bits() roundtrip failed for {:?}", entity);
+    }
+}
+
+/// PROTOCOL_VERSION constant matches snapshot version field
+#[test]
+fn protocol_version_constant_matches_snapshot() {
+    let snapshot = test_snapshot(0, vec![]);
+    assert_eq!(snapshot.version, PROTOCOL_VERSION);
+    assert_eq!(PROTOCOL_VERSION, 4, "bump this assertion when protocol version changes");
+}
+
 /// All FacingDirection variants round-trip
 #[test]
 fn all_facing_direction_variants_roundtrip() {