Stop handler now calls cancel_operation() on the device bridge,
which sets a threading.Event that interrupts the 2s-timeout serial
read loop in send_with_timeout(). InterruptedError is caught
separately to prevent falling back to software sweep on cancel.
disconnect() uses acquire(timeout=5) with force-close fallback
instead of blocking lock acquisition — prevents deadlock when a
stuck worker holds the serial lock during shutdown.
Add 3 Textual async tests (pytest-asyncio) to verify Stop behavior:
firmware sweep stop, software sweep stop, and sweep restart.
Ryan Malloy
e7e71c47d7