Preserve fractional frame rates when encoding SDR video

[Syed Osama Ali Shah (Osamaali313)]

Summary

encode_video writes the SDR H.264 stream with a truncated integer frame rate, so fractional rates are silently wrong.

packages/ltx-pipelines/src/ltx_pipelines/utils/media_io/encode.py:

stream = container.add_stream("libx264", rate=int(fps), options={"crf": str(crf), "preset": preset})

int(fps) truncates 29.97 → 29, 23.976 → 23, 59.94 → 59. The output file is then tagged/timed at the wrong rate, so it plays ~1.6–4.2% too slow and any muxed audio drifts progressively out of sync.

This is reached on the normal path: --frame-rate is a float CLI argument (utils/args.py, "Frame rate of the generated video (fps)") and every generation pipeline (distilled, ti2vid_*, a2vid_two_stage, ic_lora, keyframe_interpolation, …) passes args.frame_rate straight to encode_video, so --frame-rate 29.97 yields a 29 fps file.

Fix

Use Fraction(fps).limit_denominator(1000) for the stream rate (widening the fps parameter to float). This matches what the codebase already does everywhere else:

• encode_video's own HDR branch forwards fps=float(fps).
• media_io/exr.py uses add_stream("libx264", rate=Fraction(frame_rate).limit_denominator(1000)) — the same codec — with the docstring "so playback matches the input timing."
• ltx_core/color/hlg.py uses Fraction(fps).limit_denominator(1000) for libx265.

The SDR branch was the lone outlier. Integer rates are unchanged (Fraction(24.0).limit_denominator(1000) == 24).

I also removed the int(src.fps) pre-truncation at the retake and dubit call sites (src.fps is already a float from float(video_stream.average_rate)), so source-fps flows keep their true rate too.

Verification

Ran the real encoding path through PyAV (the same add_stream + frame-encode idiom encode_video uses):

fps input      : 29.97
RED  (int fps) : output average_rate = 29        (29.0)
GREEN(Fraction): output average_rate = 2997/100  (29.97)
24.0 fps       : output average_rate = 24         (no regression)

ruff check and ruff format are clean on the changed files.

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