fix(simulation): address #843 review — seeding, atomics, API consistency

Review fixes from Hoshe + Tyre:
- EntityRng seeding: splitmix64(seed) ^ splitmix64(id) instead of
  splitmix64(seed + id) — eliminates collision class where adjacent
  seeds produce identical streams
- AtomicBool ordering: Relaxed → SeqCst for shutdown flag (correct
  on weakly-ordered architectures)
- Worker Drop: join handles instead of detaching threads
- Normalize stub API: remove ChunkGenWorker convenience wrappers,
  use .pool consistently across all 3 workers
- trigger_monologue: downgrade &mut to shared refs (no-op anchor
  was blocking parallel systems)
- Remove dead SimRng inserts from migrated monologue tests
- Document determinism gap on poll_worker_results
- Document bevy_tasks/rayon dep rationale in Cargo.toml

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/src/simulation/monologue.rs

@@ -588,11 +588,12 @@ fn has_hear_sound_event(
 pub fn trigger_monologue(
     _time: Res<SimulationTime>,
     _query: Query<
-        (&TilePosition, &mut MonologueState, &mut MonologueBuffer),
+        (&TilePosition, &MonologueState, &MonologueBuffer),
         With<PlayerCharacter>,
     >,
 ) {
     // Intentional no-op — kept as schedule ordering anchor. See doc comment.
+    // Uses shared refs (not &mut) to avoid blocking parallel systems.
 }
 
 // ---------------------------------------------------------------------------
@@ -790,7 +791,7 @@ mod tests {
     fn setup_anomaly_world() -> World {
         let mut world = World::new();
         world.init_resource::<SimulationTime>();
-        world.insert_resource(SimRng::new(42));
+        // SimRng no longer needed — migrated systems use EntityRng
         world
     }
 
@@ -991,7 +992,7 @@ mod tests {
     fn setup_recognition_world() -> World {
         let mut world = World::new();
         world.init_resource::<SimulationTime>();
-        world.insert_resource(SimRng::new(42));
+        // SimRng no longer needed — migrated systems use EntityRng
         world
     }
 
@@ -1353,7 +1354,7 @@ mod tests {
     fn setup_event_world() -> World {
         let mut world = World::new();
         world.init_resource::<SimulationTime>();
-        world.insert_resource(SimRng::new(42));
+        // SimRng no longer needed — migrated systems use EntityRng
         world.init_resource::<ObservationEventQueue>();
         world.init_resource::<SoundEventQueue>();
         world.init_resource::<PostConversationQueue>();
@@ -1923,7 +1924,7 @@ mod tests {
     fn setup_contradiction_world() -> bevy_ecs::world::World {
         let mut world = bevy_ecs::world::World::new();
         world.init_resource::<SimulationTime>();
-        world.insert_resource(SimRng::new(42));
+        // SimRng no longer needed — migrated systems use EntityRng
         world.insert_resource(ContradictionDetectedQueue::default());
         world.init_resource::<EntityRegistry>();
         world