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/mkv.rs

@@ -1414,7 +1414,12 @@ impl<W: Write + Seek> MkvMuxer<W> {
         // outside every clip's IN/OUT marks, which only a seam-plan-driven
         // title can report. Dropping it is the point: emitting it is what put
         // duplicate content on the timeline at a join.
-        let Some(pts_ns) = self.continuity.map(pts_ns, drives_epoch, track_idx) else {
+        let Some(pts_ns) = self.continuity.map(
+            pts_ns,
+            drives_epoch,
+            track_idx,
+            self.track_is_video.get(track_idx).copied().unwrap_or(false),
+        ) else {
             return Ok(());
         };
         let raw_ticks = pts_ns / TIMESTAMP_SCALE_NS;