Build a Smaller Quinn

1. Build a deterministic Connection state machine accepting Datagram and Timeout events and yielding Transmit and StreamEvent values.
2. Test it with a simulated clock and two in-memory endpoints.
3. Add an async UDP driver that feeds datagrams into the same core.
4. Route connection IDs from one socket to several connection states.
5. Expose RecvStream and SendStream futures with per-stream wakers.
6. Add stream and connection flow-control credit.
7. Add retransmission deadlines and a simple congestion window.
8. Bound work per driver poll and test that a hot connection cannot starve one quiet connection.
9. Implement close, reset, drop behavior, and endpoint drain.

Production Quinn additionally supplies the full QUIC/TLS specifications, packet protection, migration, ECN, MTU discovery, 0-RTT, datagrams, batching, platform socket tuning, runtime adapters, fuzzing, and extensive simulations.