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/codec/pgs.rs

@@ -223,6 +223,7 @@ mod tests {
             pts,
             dts: None,
             data,
+            discontinuity: false,
         }
     }