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/workers/pool.rs

@@ -54,7 +54,7 @@ pub struct BackgroundWorkerPool<Req, Resp> {
     tx: Sender<WorkRequest<Req>>,
     rx: Receiver<(Resp, DeliveryStrategy)>,
     shutdown: Arc<AtomicBool>,
-    _workers: Vec<JoinHandle<()>>,
+    workers: Vec<JoinHandle<()>>,
 }
 
 impl<Req, Resp> BackgroundWorkerPool<Req, Resp>
@@ -84,7 +84,7 @@ where
             let handle = thread::Builder::new()
                 .name(format!("worker-{id}"))
                 .spawn(move || {
-                    while !shutdown.load(Ordering::Relaxed) {
+                    while !shutdown.load(Ordering::SeqCst) {
                         match rx.recv_timeout(std::time::Duration::from_millis(100)) {
                             Ok(work) => {
                                 let strategy = work.strategy;
@@ -107,7 +107,7 @@ where
             tx: req_tx,
             rx: resp_rx,
             shutdown,
-            _workers: workers,
+            workers,
         }
     }
 
@@ -146,13 +146,16 @@ where
 
     /// Signal all workers to shut down gracefully.
     pub fn shutdown(&self) {
-        self.shutdown.store(true, Ordering::Relaxed);
+        self.shutdown.store(true, Ordering::SeqCst);
     }
 }
 
 impl<Req, Resp> Drop for BackgroundWorkerPool<Req, Resp> {
     fn drop(&mut self) {
-        self.shutdown.store(true, Ordering::Relaxed);
-        // Workers will exit on next recv_timeout cycle (100ms max)
+        self.shutdown.store(true, Ordering::SeqCst);
+        // Join all worker threads — wait for clean exit (100ms max per thread)
+        for handle in self.workers.drain(..) {
+            let _ = handle.join();
+        }
     }
 }