v2.35.1 — extract_frames (source-safe still extraction)

• Added media_pool_item(action="extract_frames", clip_id, timestamps, output_dir?) extracts still JPEGs from a clip's source media at the given timestamps (seconds) via ffmpeg. Source-safe: it reads the source and writes only to a scratch output directory — it never modifies, transcodes, or proxies the source.

Closes a read/write-symmetry gap (the analysis sampler existed internally but had no standalone frame-extraction tool). Live-validated on DaVinci Resolve Studio 21.0.0.