fix(drone): harden RFDetector threading, subprocess lifecycle, and frequency accuracy

- Replace _running bool with threading.Event for correct cross-thread visibility
- Add _proc_lock to guard _rtl_proc/_hackrf_proc across worker/main threads
- Use register_process + safe_terminate (pipe close + SIGKILL fallback on timeout)
- Compute HackRF frequency as band midpoint (hz_low+hz_high)//2, not hz_low
- Guard start() for idempotency — double-call no longer leaks threads

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

tests/test_drone_rf_detector.py

@@ -56,7 +56,7 @@ def test_hackrf_sweep_line_emits_observation(detector):
     obs = q.get_nowait()
     assert isinstance(obs, RFObservation)
     assert obs.hardware == "HACKRF"
-    assert obs.frequency_hz == hz_low
+    assert obs.frequency_hz == (hz_low + hz_high) // 2
     assert obs.rssi < 0