Round 5: fix the gates added in round 4, and two placement holes

The zero-frame check ran before the seam gate, and its error is
classified as a skippable nav stub — so a title the plan dropped
ENTIRELY was reported as an empty stub and an all-titles rip would omit
a real feature and finish the rest at exit 0. The seam case is decided
first now, with a code that is not skippable.

The demux sink read a frame's track kind out of the FILTERED slot, which
is empty for a class the export drops. On an audio:// or sub:// export
the video track was therefore called non-video and handed the permissive
crossing rule — the same defect round 4 fixed for a Dolby Vision layer,
reintroduced one file over. Video tracks are now recorded before the kind
filter, beside the primary-video reference that exists for this reason.

Its frame counter counted frames PLACED, not written, while its name and
doc claimed otherwise. Renamed and documented for what it is, including
that it cannot see a single lost track among many.

Placement: files in a subdirectory of VIDEO_TS were never given data.
They were declared at full size with no extents, so they appeared in the
tree and read as nothing. The same folder under BDMV was always placed
correctly. And the duplicate title-set guard keyed on the constraint maps,
so an IFO declaring no offsets inserted nothing and a colliding second IFO
went undetected — it keys on the groups seen now.

Display for SeamPlanDroppedMost and ShortImageRead discarded their
payloads, and four new variants were missing from the code-uniqueness
test.

src/mux/timeline.rs

@@ -1022,7 +1022,6 @@ mod tests {
     fn a_second_video_track_keeps_the_reorder_safe_window() {
         let clips = seamless_branching_clips();
         let mut plan = SeamPlan::from_clips(&clips).expect("plan");
-        let c0_in = mpls_ticks_to_ns(clips[0].in_time);
         let c0_out = mpls_ticks_to_ns(clips[0].out_time);
         let c1_in = mpls_ticks_to_ns(clips[1].in_time);
 
@@ -1040,7 +1039,6 @@ mod tests {
             base - 42_000_000,
             "an enhancement layer's reorder dip must not be read as a join"
         );
-        let _ = c0_in;
     }
 
     /// A clip table that is not one advancing clock must fall back to