fix: isolate video codec runtime from capture

[abrichr]

Summary

• remove PyAV from the core/runtime dependency closure while keeping video enabled by default
• stage lossless timestamped PNG frames and encode transactionally through a separately provisioned FFmpeg process
• resolve Desktop-managed FFmpeg/ffprobe, explicit developer paths, or PATH without downloading or bundling codec binaries
• perform a real encode plus PNG-decode/select probe before listeners start, with verified automatic encoder choice and portable mpeg4/yuv420p fallback
• preserve nearest-frame-within-tolerance extraction and video metadata through ffprobe
• retain staged frames on encoding failure; atomically publish and delete staging only after verification

Verification

• Python 3.12 macOS unit suite: 312 passed, 8 deselected
• Python 3.10 macOS unit suite: 312 passed, 8 deselected
• focused video/import/API suite: 56 passed
• exact managed LGPL runtime smoke: mpeg4 encode, yuv420p, PNG verification, ffprobe metadata, nearest-frame extraction passed with the 5.4 MB FFmpeg 8.1.2 pair
• ruff check openadapt_capture: passed
• wheel and sdist release-boundary verifier: passed
• clean installed-wheel import: passed; av absent from dependencies and modules

Artifact boundary

The wheel and sdist contain no PyAV dependency and no FFmpeg, ffprobe, libavcodec/libavformat, x264, or x265 binary. Desktop provisions the separately distributed runtime and supplies absolute executable paths.