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/systems.rs

@@ -12,16 +12,25 @@ use super::stubs::{ChunkGenWorker, NpcPrepWorker, OffscreenTickWorker};
 
 /// PreInput: poll all worker pools for completed results.
 ///
-/// Incorporates results into the world state. Currently logs completions
-/// at debug level — real incorporation logic (spawning chunks, activating
-/// NPCs) plugs in at Phase 5+.
+/// Currently logs completions at debug level — real incorporation logic
+/// (spawning chunks, activating NPCs) plugs in at Phase 5+.
+///
+/// **DETERMINISM GAP (D-010):** Results are incorporated at the tick they
+/// happen to be ready, not at a deterministic tick. Two runs with the same
+/// seed may incorporate a chunk at tick 500 vs tick 502 depending on CPU
+/// load — diverging world state from that point.
+///
+/// This is acceptable while all handlers are no-ops (no simulation state
+/// is modified). Before any handler produces real state changes, implement
+/// a `WorkerResultLog` resource that records `(tick, worker_type, result)`.
+/// Replay reads from the log instead of polling channels. See plan #843.
 pub fn poll_worker_results(
     chunk_worker: Option<Res<ChunkGenWorker>>,
     npc_worker: Option<Res<NpcPrepWorker>>,
     offscreen_worker: Option<Res<OffscreenTickWorker>>,
 ) {
     if let Some(worker) = chunk_worker {
-        for (result, _strategy) in worker.drain() {
+        for (result, _strategy) in worker.pool.drain() {
             tracing::debug!(
                 chunk_x = result.chunk_x,
                 chunk_y = result.chunk_y,