feat(simulation): background worker pool infrastructure (#843 Part C)

Generic BackgroundWorkerPool<Req, Resp> with crossbeam channels, closure
handlers, and 3 delivery strategies (Fallback, GracefulDegrade, ModalLock).

Stub workers registered as Bevy resources:
  - ChunkGenWorker (2 threads) — terrain/props/navmesh generation
  - NpcPrepWorker (1 thread) — pre-compute NPC state for incoming areas
  - OffscreenTickWorker (1 thread) — advance NPCs outside active tier

Tick loop integration:
  - PreInput: poll_worker_results drains completed work
  - PostSnapshot: push_worker_requests queues new work (no-op until Phase 5)

Handlers are stubs — real computation plugs in when the phases that need
them arrive. The infrastructure (channels, threads, push/poll, shutdown)
is real and tested.

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

server/src/lib.rs

@@ -11,6 +11,7 @@ pub mod simulation;
 pub mod storyteller;
 pub mod tick_phases;
 pub mod voice;
+pub mod workers;
 // test_world::reset is always compiled (used by simulation::input).
 // Room definitions, constants, and setup_gauntlet are gated behind
 // the "gauntlet" feature (default-on) to allow stripping from release builds.