Fix CTS=0 in fragmented fMP4 with multiple tracks (#317)

* Fix CTS=0 in fragmented fMP4 with multiple tracks

When muxing fragmented MP4 with both video and audio tracks,
compositionTimeOffset (CTS) was zero for all video samples, causing
B-frame content to display in decode order instead of presentation
order (visible judder).

Root cause: During finalize(), interleaveSamples(true) triggers
finalizeFragment() via the cross-track keyframe check in
addSampleToTrack. This writes the trun box before processTimestamps
has computed correct decodeTimestamp values, so CTS = PTS - DTS = 0.

Fix: Call processTimestamps() for all tracks at the start of
finalizeFragment(). This is safe because processTimestamps is a no-op
when the timestampProcessingQueue is empty.

Video-only fragmented muxing was unaffected because the single-track
case never triggers the cross-track keyframe check during
interleaveSamples.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

* Fix the problem at the actual root

* Oopsie doopsie

---------

Co-authored-by: Claude Opus 4.6 <[email protected]>
Co-authored-by: Vanilagy <[email protected]>

test/node/isobmff-muxer.test.ts

@@ -61,3 +61,46 @@ test('ISOBMFF muxer internally converts ADTS to AAC', async () => {
 
 	expect(count).toBe(4557);
 });
+
+test('Fragmented fMP4 with video+audio preserves B-frame CTS', async () => {
+	using input = new Input({
+		source: new FilePathSource(path.join(__dirname, '../public/video.mp4')),
+		formats: ALL_FORMATS,
+	});
+
+	const videoTrack = await input.getPrimaryVideoTrack();
+	const audioTrack = await input.getPrimaryAudioTrack();
+	assert(videoTrack);
+	assert(audioTrack);
+
+	const originalVideoSink = new EncodedPacketSink(videoTrack);
+	const originalTimestamps: number[] = [];
+	for await (const packet of originalVideoSink.packets()) {
+		originalTimestamps.push(packet.timestamp);
+	}
+
+	const output = new Output({
+		format: new Mp4OutputFormat({ fastStart: 'fragmented' }),
+		target: new BufferTarget(),
+	});
+
+	const conversion = await Conversion.init({ input, output, showWarnings: false });
+	await conversion.execute();
+
+	using outputAsInput = new Input({
+		source: new BufferSource(output.target.buffer!),
+		formats: ALL_FORMATS,
+	});
+
+	const outputVideoTrack = await outputAsInput.getPrimaryVideoTrack();
+	assert(outputVideoTrack);
+
+	const videoSink = new EncodedPacketSink(outputVideoTrack);
+
+	const timestamps: number[] = [];
+	for await (const packet of videoSink.packets()) {
+		timestamps.push(packet.timestamp);
+	}
+
+	expect(timestamps).toEqual(originalTimestamps);
+});