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/rng.rs

@@ -52,7 +52,9 @@ impl EntityRng {
     /// Uses splitmix64 mixing to combine the two seeds with good avalanche
     /// properties — avoids correlated sequences for entities with similar IDs.
     pub fn from_seed_and_id(world_seed: u64, stable_id: u64) -> Self {
-        let mixed = splitmix64(world_seed.wrapping_add(stable_id));
+        // Mix each input independently then XOR — avoids the collision class
+        // where (seed=0, id=N) == (seed=1, id=N-1) that wrapping_add creates.
+        let mixed = splitmix64(world_seed) ^ splitmix64(stable_id);
         Self {
             rng: ChaCha20Rng::seed_from_u64(mixed),
         }
@@ -91,4 +93,33 @@ mod tests {
         let val2: u32 = rng2.rng.random();
         assert_ne!(val1, val2);
     }
+
+    #[test]
+    fn entity_rng_same_seed_same_id_same_sequence() {
+        let mut a = EntityRng::from_seed_and_id(42, 7);
+        let mut b = EntityRng::from_seed_and_id(42, 7);
+        let va: Vec<u32> = (0..100).map(|_| a.rng.random()).collect();
+        let vb: Vec<u32> = (0..100).map(|_| b.rng.random()).collect();
+        assert_eq!(va, vb);
+    }
+
+    #[test]
+    fn entity_rng_same_seed_different_id_different_sequence() {
+        let mut a = EntityRng::from_seed_and_id(42, 0);
+        let mut b = EntityRng::from_seed_and_id(42, 1);
+        let va: u32 = a.rng.random();
+        let vb: u32 = b.rng.random();
+        assert_ne!(va, vb);
+    }
+
+    #[test]
+    fn entity_rng_adjacent_seeds_no_collision() {
+        // Verify (seed=0, id=N) != (seed=1, id=N-1) — the collision class
+        // that wrapping_add would create.
+        let mut a = EntityRng::from_seed_and_id(0, 5);
+        let mut b = EntityRng::from_seed_and_id(1, 4);
+        let va: u32 = a.rng.random();
+        let vb: u32 = b.rng.random();
+        assert_ne!(va, vb);
+    }
 }