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/tests/bridge_ipc.rs

@@ -39,7 +39,6 @@ fn snapshot_roundtrip_over_unix_socket() {
                 day: 0,
                 time_of_day: 0,
                 day_phase: DayPhase::Morning,
-                paused: false,
                 tick_rate: TickRate::Full,
             },
             player_facing: FacingDirection::North,

server/tests/bridge_tcp.rs

@@ -25,7 +25,6 @@ fn snapshot_roundtrip_over_tcp() {
                 day: 0,
                 time_of_day: 0,
                 day_phase: DayPhase::Morning,
-                paused: false,
                 tick_rate: TickRate::Full,
             },
             player_facing: FacingDirection::North,

server/tests/game_loop.rs

@@ -68,7 +68,7 @@ fn player_moves_north_through_full_pipeline() {
     // v2 fields populated
     assert_eq!(snapshot.game_time.day, 0);
     assert_eq!(snapshot.game_time.day_phase, settled_reach_server::simulation::time::DayPhase::Morning);
-    assert!(!snapshot.game_time.paused);
+    assert_eq!(snapshot.game_time.tick_rate, settled_reach_server::simulation::time::TickRate::Full);
 
     let player_entity = &snapshot.entities[0];
     // Player started at (16, 16, 0), moved north (y-1) to (16, 15, 0)

server/tests/gen_fixtures.rs

@@ -24,7 +24,6 @@ fn fixture_snapshot(tick: u64, entities: Vec<VisibleEntity>) -> ObserverSnapshot
             day: 0,
             time_of_day: 0,
             day_phase: DayPhase::Morning,
-            paused: false,
             tick_rate: TickRate::Full,
         },
         player_facing: FacingDirection::North,
@@ -158,7 +157,6 @@ fn generate_msgpack_fixtures() {
             day: 1,
             time_of_day: 720,
             day_phase: DayPhase::Evening,
-            paused: false,
             tick_rate: TickRate::Full,
         },
         player_facing: FacingDirection::Southeast,

server/tests/serialization.rs

@@ -13,7 +13,6 @@ fn test_snapshot(tick: u64, entities: Vec<VisibleEntity>) -> ObserverSnapshot {
             day: 0,
             time_of_day: 0,
             day_phase: DayPhase::Morning,
-            paused: false,
             tick_rate: TickRate::Full,
         },
         player_facing: FacingDirection::North,
@@ -127,8 +126,9 @@ fn all_fixtures_deserialize() {
         let bytes = fs::read(&path).unwrap_or_else(|_| panic!("read fixture {}", name));
 
         if name.starts_with("snapshot") {
-            rmp_serde::from_slice::<ObserverSnapshot>(&bytes)
+            let snap = rmp_serde::from_slice::<ObserverSnapshot>(&bytes)
                 .unwrap_or_else(|e| panic!("deserialize snapshot fixture {}: {}", name, e));
+            assert_eq!(snap.version, 4, "fixture {} has wrong version", name);
         } else if name.starts_with("input_batch") {
             rmp_serde::from_slice::<Vec<PlayerInput>>(&bytes)
                 .unwrap_or_else(|e| panic!("deserialize batch input fixture {}: {}", name, e));
@@ -186,7 +186,6 @@ fn snapshot_v2_fields_roundtrip() {
             day: 3,
             time_of_day: 720,
             day_phase: DayPhase::Evening,
-            paused: true,
             tick_rate: TickRate::Paused,
         },
         player_facing: FacingDirection::Southeast,
@@ -224,7 +223,7 @@ fn snapshot_v2_fields_roundtrip() {
     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);
-    assert!(decoded.game_time.paused);
+    assert_eq!(decoded.game_time.tick_rate, TickRate::Paused);
     assert_eq!(decoded.player_facing, FacingDirection::Southeast);
     assert_eq!(decoded.visible_tiles.len(), 2);
     assert_eq!(decoded.visible_tiles[0].visibility, VisibilitySector::Forward);
@@ -254,7 +253,6 @@ fn all_facing_direction_variants_roundtrip() {
                 day: 0,
                 time_of_day: 0,
                 day_phase: DayPhase::Morning,
-                paused: false,
                 tick_rate: TickRate::Full,
             },
             player_facing: dir,