Rewrite server worker from selectors to asyncio

Replace the hand-rolled selectors event loop, PollableMethodQueue pipe,
and manual keepalive murder sweep with asyncio primitives. All requests
still run synchronously in the thread pool via run_in_executor.

- Accept loops are async tasks with backpressure via asyncio.Event
- Keepalive uses asyncio.wait_for with timeout instead of deque sweep
- Graceful shutdown uses asyncio.wait + task cancellation
- Accept-loop crashes are detected and surface as worker restarts
- ~110 lines removed, no new dependencies, no new features

Author: davegaeddert