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

tests/gtest/timeline_waveform_sync_test.cpp

@@ -95,6 +95,36 @@ TEST(TimelineWaveformSync, ExtractEnvelopeUsesOnlyValidatedRanges)
 	EXPECT_DOUBLE_EQ(envelope.at(59), 0.0);
 }
 
+TEST(TimelineWaveformSync, ExtractEnvelopeReportsValidityMask)
+{
+	constexpr int kSampleRate = 48000;
+	constexpr size_t kWindowSamples = kSampleRate / 20; // 50 ms windows
+
+	AudioWaveformCache cache;
+	WritePartialWaveform(&cache, kSampleRate);
+
+	WaveformSyncClip clip;
+	clip.waveform = &cache;
+	clip.media_range = TimeRange(0, 3);
+	clip.sample_rate = kSampleRate;
+
+	QVector<bool> valid_mask;
+	const QVector<double> envelope =
+		TimelineWaveformSync::ExtractWaveformCacheEnvelope(
+			clip, kSampleRate, kWindowSamples, &valid_mask);
+
+	// One flag per envelope window
+	ASSERT_EQ(valid_mask.size(), envelope.size());
+
+	// Windows outside the validated second are flagged invalid, windows
+	// inside it are flagged valid
+	EXPECT_FALSE(valid_mask.at(0));
+	EXPECT_FALSE(valid_mask.at(59));
+	for (int i = 20; i < 40; ++i) {
+		EXPECT_TRUE(valid_mask.at(i));
+	}
+}
+
 TEST(TimelineWaveformSync, PartialCacheIsConsideredReady)
 {
 	constexpr int kSampleRate = 48000;