Add keyframe format v1 CLI export

[MuhammadBilalKhan267]

Summary

Adds a save-keyframes CLI command that exports detected scene changes using keyframe format v1.

The command supports custom filenames and output directories through CLI options and scenedetect.cfg.

Testing

Added CLI tests covering:

• Default output
• Custom filenames
• Detection with a start offset

Known limitation

VFR inputs may produce incorrect frame numbers because cuts currently store timestamps and FrameTimecode.frame_num derives a frame number using the average frame rate. Exact VFR frame ordinal support will be handled separately.

Addresses #534

[Breakthrough]

VFR inputs may produce incorrect frame numbers because cuts currently store timestamps and FrameTimecode.frame_num derives a frame number using the average frame rate. Exact VFR frame ordinal support will be handled separately.

Can we add a test case somehow for downstream usage of the file format to ensure it's valid? E.g. if ffmpeg accepts it can we run it to verify the output. I would like VFR handled in this PR if possible.

[MuhammadBilalKhan267]

I think the VFR problem deserves it's own issue. frame_num gets used in save_qp and threshold_detector as well.