Key the seam-crossing rule on reorder, not on driving epochs

A Dolby Vision enhancement layer is a second video track: it does not
drive epochs, but it does carry B-frame reorder. The crossing rule was
keyed on driving epochs, so the EL took the branch whose premise is that
the track has no reorder — and its ordinary reorder dip near the end of a
clip, which during an overlap also lands inside the next clip's range,
was read as a join. The EL was then placed on the next clip's offset, out
of step with the base-layer frame it must be co-timed with by the width
of the overlap: the same desync the per-track cursor was added to remove,
reintroduced for one track.

The property the rule actually depends on is whether a backward step can
be reorder, so it is now keyed on that. Both sinks derive it from the
track kind rather than from the epoch driver.

src/mux/demux_sink.rs

@@ -866,9 +866,18 @@ impl Stream for DemuxSink {
         // an audio ES before the video ES, in which case track 0 is audio and a
         // non-video epoch driver would ratchet the frontier on sparse/lagging PTS.
         let drives = Some(frame.track) == self.ref_video_track;
+        // Keyed on the track KIND, not on driving epochs: a second video track
+        // (a Dolby Vision enhancement layer) does not drive epochs but does
+        // carry B-frame reorder, and the seam-crossing rule differs on exactly
+        // that property.
+        let is_video = self
+            .tracks
+            .get(frame.track)
+            .and_then(|s| s.as_ref())
+            .is_some_and(|t| t.kind == TrackKind::Video);
         // 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, frame.track) else {
+        let Some(pts) = self.timeline.map(frame.pts, drives, frame.track, is_video) else {
             return Ok(());
         };
         if drives {