fix(rendering): keep late camera timestamps in the FFmpeg decoder

[yashranaway]

this is the camera-pts-late export case from the verify expectations. the ffmpeg decoder subtracted the stream's start time when labelling frames but seeked on the original timeline, so a camera file whose timestamps start 2s in played up to 60 frames ahead from the very first output frame. avassetreader already keeps the original timestamps.

now ffmpeg keeps them too, and the existing first frame hold covers the gap before the camera starts. the xfail is gone and the late start model sequences now run for ffmpeg too. five of those seeds hit the same grid rounding entry the zero start cases already have, so they get one.

🤖 Generated with Claude Code

Confidence Score: 5/5

The PR appears safe to merge; no actionable defects were found.

Summary

The FFmpeg decoder now preserves presentation timestamps instead of subtracting the stream’s start time.

• FFmpeg keeps late-start video frames on their original timeline.

_{Reviews (2) · Last reviewed commit: "Merge remote-tracking branch 'origin/mai..." · Reviewed by Greptile}

[yashranaway]

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[yashranaway]

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