mux: B1 drop-to-keyframe resync after a concealed gap

Pairs with A2 (read-path NULL-TS concealment). When the demux assembler
sees a TS continuity gap it now stamps `discontinuity` on the next
completed PES; the codec-parse stage carries that onto a per-track
ResyncGate. After a gap on an inter-coded video track the gate drops
forward to the next IRAP/IDR keyframe so no frame with a dangling
reference reaches the muxer (an ffmpeg deep scan would otherwise report
a missing-reference / non-existing-PPS error). Audio and subtitle tracks
have no cross-frame references, so the gate is a no-op there.

- ts.rs: PesPacket gains `discontinuity`; PesAssembler tracks a sticky
  pending_discontinuity flag set on CC gap / discontinuity_indicator and
  carried to the next completed/flushed PES.
- resync.rs (new): ResyncGate — per-track arm-on-gap, drop non-keyframes
  until the next keyframe disarms and resumes. Logs the resync + drop
  count once at the keyframe.
- pipelined_stream.rs: precompute per-track is_video, apply the gate in
  consume_ts. Out-of-range track index emits as-is (defensive).

Tests: ResyncGate unit tests; ts.rs gap-stamps-discontinuity; end-to-end
B1 video-drops-to-keyframe and audio-never-drops through PipelinedPesStream.

src/mux/disc.rs

@@ -723,6 +723,8 @@ impl crate::pes::Stream for DiscStream {
                             pts: ps.pts.map(|p| p as i64),
                             dts: ps.dts.map(|d| d as i64),
                             data: ps.data.clone(),
+                            // PS (DVD/CSS) path: no AACS conceal, no gap flag.
+                            discontinuity: false,
                         };
                         if let Some((_, parser)) = self.parsers.iter_mut().find(|(p, _)| *p == pid)
                         {
@@ -846,6 +848,8 @@ impl crate::pes::Stream for DiscStream {
                         pts: ps.pts.map(|p| p as i64),
                         dts: ps.dts.map(|d| d as i64),
                         data: ps.data.clone(),
+                        // PS (DVD/CSS) path: no AACS conceal, no gap flag.
+                        discontinuity: false,
                     };
 
                     if let Some((_, parser)) = self.parsers.iter_mut().find(|(p, _)| *p == pid) {