sync: masked waveform correlation, stretch sync, manual start time

- Waveform sync no longer treats uncached waveform regions as silence:
  the envelope extraction now reports a per-window validity mask and the
  correlation skips invalid windows on either side, improving accuracy
  for partially cached clips
- Add stretch/speed sync: AudioWaveformSync::EstimateStretchAndOffset
  searches a playback-rate range plus offset, and a new timeline
  context action 'Synchronize by Waveform (Adjust Speed)' applies the
  estimated rate as a clip speed change (with undo) when plain offset
  alignment is inconclusive
- Footage properties dialog gains a Source Start Time field so the
  value used by source-time sync can be viewed and edited manually
  instead of relying solely on auto-detected metadata; applied via an
  undo command, with Footage::ClearSourceStartTime() for removal
- Regression tests for masked correlation, stretch estimation, the
  envelope validity mask, and source-start-time set/clear

app/widget/timelinewidget/timelinewidget.h

@@ -109,6 +109,8 @@ public:
 
 	void SynchronizeSelectedClipsByWaveform();
 
+	void SynchronizeSelectedClipsByWaveformWithSpeed();
+
 	void GenerateProxiesForSelectedClips();
 
 	void SetSelectedClipsProxyEnabled(bool enabled);
@@ -345,6 +347,8 @@ private:
 
 	bool PasteInternal(bool insert);
 
+	void SynchronizeSelectedClipsByWaveformInternal(bool allow_speed);
+
 	TimelineAndTrackView *AddTimelineAndTrackView(Qt::Alignment alignment);
 
 	QHash<Node *, Node *>