fix: Preserve image-start samples across VIS-to-decoder boundary

VISDetector._process_window() was calling self.reset() inside the
STOP_BIT handler, wiping self._buffer before feed() could advance
past the triggering window. All audio samples buffered after the
VIS STOP_BIT (the start of the first scan line) were silently
discarded, causing the image decoder to begin decoding mid-stream
with no alignment reference. The result was every scan line being
desynchronised from the first, producing the diagonal stripes and
scrambled colours seen in decoded images.

Fix: remove the premature reset() from _process_window(). The
STOP_BIT handler now sets state=DETECTED and returns the result.
A new remaining_buffer property exposes the post-VIS samples.
_decode_audio_stream() and decode_file() capture those samples
before calling reset(), then immediately feed them into the newly
created SSTVImageDecoder so decoding begins from sample 0 of
the first sync pulse.

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

utils/sstv/vis.py

@@ -145,6 +145,16 @@ class VISDetector:
     def state(self) -> VISState:
         return self._state
 
+    @property
+    def remaining_buffer(self) -> np.ndarray:
+        """Unprocessed samples left after VIS detection.
+
+        Valid immediately after feed() returns a detection result and before
+        reset() is called. These samples are the start of the SSTV image and
+        must be forwarded to the image decoder.
+        """
+        return self._buffer
+
     def feed(self, samples: np.ndarray) -> tuple[int, str] | None:
         """Feed audio samples and attempt VIS detection.
 
@@ -288,9 +298,22 @@ class VISDetector:
 
             if self._tone_counter >= self._bit_windows:
                 result = self._validate_and_decode()
-                self.reset()
+                # Do NOT call reset() here. self._buffer still holds samples
+                # that arrived after the STOP_BIT window — those are the very
+                # first samples of the image. Wiping the buffer here causes all
+                # of them to be lost, making the image decoder start mid-stream
+                # and producing garbled/diagonal output.
+                # feed() will advance past the current window, leaving
+                # self._buffer pointing at the image start. The caller must
+                # read remaining_buffer and then call reset() explicitly.
+                self._state = VISState.DETECTED
                 return result
 
+        elif self._state == VISState.DETECTED:
+            # Waiting for caller to call reset() after reading remaining_buffer.
+            # Don't process any windows in this state.
+            pass
+
         return None
 
     def _decode_bit(self, samples: np.ndarray) -> int: