Add MP4 intraframe format for frame-by-frame encoding

[Copilot]

The existing MP4 format uses mp4v codec (MPEG-4 Part 2) with interframe compression, making frame-by-frame work impossible. Added MP4 (I-Frame) format for true intraframe encoding.

Changes

• New format option: "MP4 (I-Frame)" using H.264 with keyint=1:scenecut=0 to force all I-frames
• VideoBackgroundWorker: Added video_format parameter to pass format context to FFmpeg merge
• FFmpeg encoding logic: Apply x264 parameters conditionally based on format selection
• Tests: Added format validation, codec mapping, and intraframe parameter tests with shared constants

Format comparison

Format	Encoding	Codec	Frame Independence
MP4	Interframe	mp4v	❌ No
MP4 (I-Frame)	Intraframe	H.264	✅ Yes
AVI	Intraframe	MJPEG	✅ Yes
MKV	Intraframe	FFV1	✅ Yes

Implementation

# node_video_writer.py - Format configuration
format_config = {
    'MP4 (I-Frame)': {'ext': '.mp4', 'codec': 'H264'}
}

# video_worker.py - FFmpeg parameters
if video_format == 'MP4 (I-Frame)':
    vcodec = 'libx264'
    vcodec_params = 'keyint=1:scenecut=0'  # All I-frames

Documentation

• node/VideoNode/README_VideoWriter.md - Format details and use cases
• node/VideoNode/README_VideoWriter_FR.md - French translation

Original prompt

est ce que je peux faire du frame par frame avec l'option mp4 du videowriter ?

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