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/input.rs

@@ -102,17 +102,16 @@ fn apply_move(
     dx: i32,
     dy: i32,
 ) {
-    if let Ok((entity, pos)) = player_query.single() {
-        commands.entity(entity).insert(MoveIntent {
-            target: TilePosition::new(pos.x + dx, pos.y + dy, pos.z),
-        });
-        // Update facing direction based on movement (D-015 vision cone)
-        commands
-            .entity(entity)
-            .insert(Facing(facing_from_delta(dx, dy)));
-    } else {
-        tracing::warn!("No player entity found for movement input");
-    }
+    let (entity, pos) = player_query
+        .single()
+        .expect("PlayerCharacter entity must exist when processing input");
+    commands.entity(entity).insert(MoveIntent {
+        target: TilePosition::new(pos.x + dx, pos.y + dy, pos.z),
+    });
+    // Update facing direction based on movement (D-015 vision cone)
+    commands
+        .entity(entity)
+        .insert(Facing(facing_from_delta(dx, dy)));
 }
 
 #[cfg(test)]
@@ -220,7 +219,8 @@ mod tests {
     }
 
     #[test]
-    fn process_input_no_player_no_panic() {
+    #[should_panic(expected = "PlayerCharacter entity must exist")]
+    fn process_input_no_player_panics() {
         let mut world = bevy_ecs::world::World::new();
         world.insert_resource(InputQueue::default());
         world.insert_resource(SimulationTime::default());
@@ -232,7 +232,6 @@ mod tests {
 
         let mut schedule = bevy_ecs::schedule::Schedule::default();
         schedule.add_systems(process_player_input);
-        // Should not panic
         schedule.run(&mut world);
     }