refactor(server): address PR #16 review feedback

Hoshe + Tyre review items:
- Use StableId consistently for wire entity_id (H4) across observer,
  observation, interpretation, and interaction systems
- Make NearbyInteractionBuffer.interactions private with take() (H1/H20)
- Add system ordering constraint for compute_nearby_interactions (H5)
- Panic on missing PlayerCharacter in input processing (H2)
- Remove redundant paused field from GameTime (Tyre8)
- Remove #[serde(default)] from nearby_interactions (H3)
- Change NearbyInteraction.distance from f32 to u32 (H8)
- Add sort stability for equal verb priorities (H6)
- Scope constants to pub(crate) (H7)
- Add debug_assert for last_observed_tick ordering (H10)
- Strengthen unregistered entity handling to debug_assert + error (H11)
- Document fractional tick accumulation (Tyre9)
- Extract collect_remembered_entities helper (Tyre2/H17)
- Add half_rate_no_drift_over_10000_frames test (H14)
- Add mid-range and deterministic sort tests (H15)
- Add fixture version assertion (H16)
- Regenerate msgpack fixtures for wire format changes

146 unit + 19 integration tests pass, zero clippy warnings.

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

server/src/simulation/time.rs

@@ -92,6 +92,11 @@ impl SimulationTime {
 
 /// Advance the simulation tick based on current tick rate.
 /// Full: +1 every frame. Half: +1 every 2 frames. Paused: no advance.
+///
+/// Uses fractional accumulation: each frame adds `tick_rate.scale()` to an
+/// internal accumulator. When it reaches 1.0, a tick fires and the accumulator
+/// subtracts 1.0. This ensures Half rate produces exactly N/2 ticks over N
+/// frames with no floating-point drift (0.5 is exactly representable in f32).
 pub fn advance_tick(mut time: ResMut<SimulationTime>) {
     let scale = time.tick_rate.scale();
     if scale <= 0.0 {
@@ -193,4 +198,19 @@ mod tests {
         let time = SimulationTime { tick: 3 * MINUTES_PER_DAY * TICKS_PER_GAME_MINUTE + 100, ..Default::default() };
         assert_eq!(time.day(), 3);
     }
+
+    #[test]
+    fn half_rate_no_drift_over_10000_frames() {
+        let mut world = bevy_ecs::world::World::new();
+        world.insert_resource(SimulationTime { tick_rate: TickRate::Half, ..Default::default() });
+        let mut schedule = bevy_ecs::schedule::Schedule::default();
+        schedule.add_systems(advance_tick);
+
+        for _ in 0..10_000 {
+            schedule.run(&mut world);
+        }
+
+        // 10,000 frames at Half rate (0.5) should yield exactly 5,000 ticks
+        assert_eq!(world.resource::<SimulationTime>().tick, 5_000);
+    }
 }