fix(demux): reconstruct clamped Matroska H264 timestamps

[oxyc]

Summary

Reconstruct clamped timestamps in narrowly identified CFR H.264 Matroska streams. The affected files contain duplicated timestamp slots and missing CFR slots,
producing alternating 0 ms / 80 ms presentation intervals in 25 fps video.

What changed

• Detect the affected shape using an exact whole-tick cadence, valid picture-order ranks, and multiple corroborating one-frame clamps.
• Reconstruct PTS and DTS from the IDR anchor, picture order, and decode order.
• Reanchor after seeks and IDRs, and fail closed on overflow or any unexpected timestamp shape.
• Add regression coverage for missing DTS and independent one-frame-forward PTS/DTS clamps.

Test plan

• Device / OS: Apple TV 4K (3rd generation, AppleTV14,1), tvOS 27.0 (24J361)

• Source media (container / video codec + profile / audio / HDR-DV): Matroska, 1920×1080 at 25 fps / H.264 High@L4.0 / AC-3 48 kHz stereo / SDR

• Result: The affected stream changed from persistent alternating 0 ms / 80 ms decoded-frame intervals to a steady 40 ms cadence and was visually smooth. Native
  playback requested 25 fps on the 50 Hz display with no rebuffering.

• Reproducer: synthetic 25 fps H.264 Matroska fixture, with packets 3 and 7 clamped one frame forward to create duplicate and
  missing timestamp slots.

• Focused tests: all 13 Matroska timestamp tests pass.

• Full suite: 4,347 of 4,348 tests pass. One unrelated slow-peer networking test timed out twice.

Build the synthetic fixture with FFmpeg

ffmpeg -f lavfi -i testsrc2=size=320x180:rate=25 -t 1 \
  -c:v libx264 -profile:v high -level:v 4.0 -pix_fmt yuv420p \
  -x264-params 'keyint=25:min-keyint=25:scenecut=0:bframes=3:b-adapt=0' \
  -an healthy.mkv

ffmpeg -i healthy.mkv -map 0:v:0 -c copy \
  -bsf:v "setts=pts='if(eq(N,3)+eq(N,7),PTS+0.04/TB,PTS)':dts='if(eq(N,3)+eq(N,7),DTS+0.04/TB,DTS)'" \
  clamped.mkv

Checklist

• CHANGELOG.md updated
• Commit messages follow Conventional Commits (feat(...), fix(...), chore(...))
• The fix lives in the engine, not in a host-side workaround
• Public API changes are intentional and documented — no public API changes