fix _check_churn self-deadlock — remove redundant known_lock nesting

on_departure() holds known_lock when calling _check_churn(). When churn
threshold is hit, the nested `with known_lock:` inside _check_churn tried
to reacquire an already-held non-reentrant lock → permanent deadlock.

This killed the production process on the OPi Zero 3 after ~15 hours of
runtime on 2026-04-12 at ~02:12 UTC. infrastructure_ips.add() is safe
without the inner lock since the caller already holds known_lock.

Also: reset _churn_tracker between tests to prevent cross-test state
accumulation that triggered the churn threshold in test isolation.

net_alerter/test_net_alerter.py

@@ -39,6 +39,7 @@ def cleanup_after_each_test():
     net_alerter.departure_timers.clear()
     net_alerter.known_devices.clear()
     net_alerter.last_departed_time.clear()
+    net_alerter._churn_tracker.clear()
     cleanup_flap_state()
     # Force garbage collection to ensure threads are cleaned up
     import gc