Video: ProRes container recognition and decode

[justin13888]

VideoCodecId::ProRes exists in the API and returns a matchable VideoError::UnsupportedCodec. Nothing decodes it, and no container currently surfaces a ProRes track.

Two parts:

1. Container — rawshift-video-isobmff maps sample entries through mp4_atom::Codec, which does not model the ProRes 4CCs (ap4h, apch, apcn, apcs, apco), so a ProRes track currently falls through to Track::Other. Either upstream them to mp4-atom or handle the raw 4CCs directly.
2. Decode — unlike H.264 and HEVC, ProRes is a plausible candidate for a clean-slate safe-Rust decoder: the bitstream is publicly specified (SMPTE RDD 36) and intra-only, so docs/SUPPORT.md records it as undecided rather than never. Hardware ProRes decode also exists on Apple silicon via VideoToolbox, which would come free with Hardware decode: VideoToolbox backend (macOS/iOS, HEVC + AV1) #28.

ProRes is on the roadmap for iPhone Pro and professional editing workflows, so this is worth doing — but it is a genuine decision about whether rawshift wants to own a codec implementation.

Follow-up to #39.