perf(ipc): replace Mutex with channel-based writer task

Eliminate Arc<Mutex> contention in IPC listener by introducing a
dedicated writer task that exclusively owns the socket write half.

- Add WriteCommand enum for all write operations (Response, Heartbeat,
  Push, SubscriptionResponse, DiscoveryResponse, StreamFrame)
- Add run_writer_task that processes commands from an MPSC channel
- Refactor handle_connection to spawn writer task and use channel
- Update push forwarder to send through channel instead of locking
- Extract process_frame and run_connection_loop for better structure

This eliminates lock contention between the main request loop and push
notification forwarder, improving throughput under concurrent load.

Author: rrrodzilla