Core no longer owns or drives OS threads. The runtime is now a single
tick-driven worker whose loop an external engine hosts and advances.
This is the cutover required before the engine seam is introduced.
Removed from core:
- Runtime::run() and its owned thread pool (spawn, park/unpark, join)
- notify_worker() and worker_threads: Vec<OnceLock<Thread>> plumbing
- Placement load-aware selector and its WorkerStats-driven next_worker()
- WorkerId newtype and the address->worker routing map; AddressMap is
now a plain AddrSet membership set
- num_threads from RuntimeConfig
Rewired for the single-worker tick API:
- python/wasm bindings, dashboard dummy node (deleted), myelin vastai
adapter, and the runtime/test suites
Cleanup folded in during review:
- prune three never-written WorkerStats counters (cross_sends,
messages_dropped, restarts)
- collapse the redundant tick_all params onto the WorkerContext handle
