Fix SSTV decoder thread lifecycle and VIS detection reliability

Three bugs preventing the live SSTV pipeline from working:

1. Race condition: self._running was set AFTER starting the decode
   thread, so the thread checked the flag, found it False, and exited
   immediately without ever processing audio.

2. Ghost running state: when the decode thread exited (e.g. rtl_fm
   died), self._running stayed True. The decoder reported as running
   but was dead, and subsequent start() calls returned without doing
   anything - permanently stuck until app restart.

3. VIS detection fragility: unclassifiable windows at tone transition
   boundaries (mixed energy from two tones) caused the state machine
   to reset from LEADER/BREAK states back to IDLE, dropping valid
   VIS headers on real signals.

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

utils/sstv/sstv_decoder.py

@@ -283,8 +283,10 @@ class SSTVDecoder:
             try:
                 freq_hz = self._get_doppler_corrected_freq_hz()
                 self._current_tuned_freq_hz = freq_hz
-                self._start_pipeline(freq_hz)
+                # Set _running BEFORE starting the pipeline so the decode
+                # thread sees it as True on its first loop iteration.
                 self._running = True
+                self._start_pipeline(freq_hz)
 
                 # Start Doppler tracking thread if enabled
                 if self._doppler_enabled:
@@ -306,6 +308,7 @@ class SSTVDecoder:
                 return True
 
             except Exception as e:
+                self._running = False
                 logger.error(f"Failed to start SSTV decoder: {e}")
                 self._emit_progress(DecodeProgress(
                     status='error',
@@ -433,7 +436,26 @@ class SSTVDecoder:
                     break
                 time.sleep(0.1)
 
-        logger.info("Audio decode thread stopped")
+        # Clean up if the thread exits while we thought we were running.
+        # This prevents a "ghost running" state where is_running is True
+        # but the thread has already died (e.g. rtl_fm exited).
+        with self._lock:
+            was_running = self._running
+            self._running = False
+            if was_running and self._rtl_process:
+                with contextlib.suppress(Exception):
+                    self._rtl_process.terminate()
+                    self._rtl_process.wait(timeout=2)
+                self._rtl_process = None
+
+        if was_running:
+            logger.warning("Audio decode thread stopped unexpectedly")
+            self._emit_progress(DecodeProgress(
+                status='error',
+                message='Decode pipeline stopped unexpectedly'
+            ))
+        else:
+            logger.info("Audio decode thread stopped")
 
     def _save_decoded_image(self, decoder: SSTVImageDecoder,
                             mode_name: str | None) -> None:

utils/sstv/vis.py

@@ -193,6 +193,8 @@ class VISDetector:
                 # Transition to BREAK; this window counts as break window 1
                 self._tone_counter = 1
                 self._state = VISState.BREAK
+            elif tone is None:
+                pass  # Ambiguous window at tone boundary — stay in state
             else:
                 self._tone_counter = 0
                 self._state = VISState.IDLE
@@ -207,6 +209,8 @@ class VISDetector:
                 # Transition to LEADER_2; this window counts
                 self._tone_counter = 1
                 self._state = VISState.LEADER_2
+            elif tone is None:
+                pass  # Ambiguous window at tone boundary — stay in state
             else:
                 self._tone_counter = 0
                 self._state = VISState.IDLE
@@ -227,6 +231,8 @@ class VISDetector:
                     self._data_bits = []
                     self._bit_accumulator = []
                     self._state = VISState.DATA_BITS
+            elif tone is None:
+                pass  # Ambiguous window at tone boundary — stay in state
             else:
                 self._tone_counter = 0
                 self._state = VISState.IDLE