- _movement_poll_loop: remove premature STOPPED_OFF check that caused the
  loop to exit after 1 iteration (gateway returns cached STOPPED_OFF before
  actuator confirms movement). External callers (_stop_movement_polling via
  updateCommeoDeviceValuesFromResponse and processEventResponse) already
  cancel the loop when the device actually stops.
- _movement_poll_loop: add try/except CancelledError + finally cleanup so
  task cancellation is handled cleanly.
- startWorker: skip re-entry when tx_task is already running to eliminate
  duplicate "Starting worker" log on every executeCommand call.
- serial_transport: add debug log "Serial RX: ..." when a complete XML
  message is flushed from the read buffer, enabling RX diagnosis.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>