feat: jump playback to 3s before the new end when the play area shrinks

When autoclip (A / back button) or a wheel scroll reduces the clip span, seek
playback to 3s before the new end and loop there so the shorter cut point can
be reviewed immediately. Growing the play area is unaffected.

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

main.py

@@ -5784,10 +5784,24 @@ class MainWindow(QMainWindow):
             eff = 1.0
         self._mpv.set_speed(eff)
 
+    def _preview_clip_end(self) -> None:
+        """Jump playback to 3s before the end of the (new) clip span and loop,
+        so a just-shrunk play area can be reviewed at its cut point."""
+        if not self._file_path:
+            return
+        end = self._cursor + self._clip_span
+        target = max(self._cursor, end - 3.0)
+        self._mpv.play_loop(self._cursor, end, resume=True)
+        self._mpv.seek(target)
+        self._timeline.set_play_position(target)
+
     def _change_clip_count(self, delta: int) -> None:
         """Wheel-scroll over the timeline adds/removes clips (clamped)."""
         spn = self._spn_clips
-        spn.setValue(max(spn.minimum(), min(spn.maximum(), spn.value() + delta)))
+        old = spn.value()
+        spn.setValue(max(spn.minimum(), min(spn.maximum(), old + delta)))
+        if spn.value() < old:               # play area got smaller
+            self._preview_clip_end()
 
     def _autoclip(self):
         """Set clip count to fit the current pause position."""
@@ -5800,7 +5814,10 @@ class MainWindow(QMainWindow):
         spread = self._spn_spread.value()
         n = int((elapsed - self._clip_dur) / spread) + 1
         n = max(1, n)
+        old_span = self._clip_span
         self._spn_clips.setValue(n)
+        if self._clip_span < old_span:      # autoclip shrank the play area
+            self._preview_clip_end()
 
     def _step_cursor(self, delta: float) -> None:
         if not self._file_path: