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/timelinewidgetwaveformsync.h

@@ -71,11 +71,15 @@ GetSelectedWaveformSyncClips(const QVector<Block *> &blocks);
  * @brief Extract a peak envelope from the validated regions of a waveform cache.
  *
  * Windows that have not been cached yet are filled with zero so that every
- * envelope stays aligned to the same absolute timeline.
+ * envelope stays aligned to the same absolute timeline; when @p valid_mask is
+ * provided it receives one flag per window marking whether the window was
+ * actually cached, allowing the correlation to skip uncached regions instead
+ * of treating them as silence.
  */
 QVector<double> ExtractWaveformCacheEnvelope(const WaveformSyncClip &clip,
 											 int sample_rate,
-											 size_t window_samples);
+											 size_t window_samples,
+											 QVector<bool> *valid_mask = nullptr);
 
 } // namespace TimelineWaveformSync