Cross a clip join per track, not on the video's frame

The first cut placed every track with the cursor the video had moved. At
an overlap join the previous clip's audio is still arriving after video
has crossed, and those tail frames sit inside both clips' mark ranges —
so they took the new clip's offset, jumped forward by the overlap, and
collided with the new clip's own audio, which the muxer's monotonic nudge
then flattened. A remux confirmed it: the timeline length was already
correct and the original symptom was still there, 169 audio packets on
the tick floor.

A track's PTS only runs forward inside a clip, so its own backward step
to the next clip's IN is its crossing. That is per track, so the cursor
is too.

src/mux/demux_sink.rs

@@ -868,7 +868,7 @@ impl Stream for DemuxSink {
         let drives = Some(frame.track) == self.ref_video_track;
         // See `MkvMuxer::write_frame`: `None` is material outside the
         // playlist's clip marks and is dropped rather than emitted.
-        let Some(pts) = self.timeline.map(frame.pts, drives) else {
+        let Some(pts) = self.timeline.map(frame.pts, drives, frame.track) else {
             return Ok(());
         };
         if drives {