feat(moq-video,libmoq): native H.264 decode (drop ffmpeg dependency)

[kixelated]

Summary

Adds a native H.264 decode path mirroring moq-video's existing native encode side, and exposes it through libmoq. This is the symmetric counterpart to how audio already works (moq-audio decodes Opus natively → moq_consume_audio_raw), and the prerequisite for removing ffmpeg from the in-tree OBS plugin (cpp/obs), which currently decodes subscribed video with libavcodec + libswscale.

This is Phase 1 (the Rust side) of that effort. The OBS plugin rewrite + CI/docs is a follow-up on top of #1787 (it can't compile against this API until it lands).

rs/moq-video — new decode module (mirror of encode)

• decode::Consumer — the direct counterpart to moq_audio::AudioConsumer: subscribes to a moq-mux H.264 track and returns raw packed-I420 decode::Frames.
• Pluggable backends behind the same trait/selector pattern as encode:
  • videotoolbox — macOS hardware (VTDecompressionSession, hand-written on the objc2-video-toolbox bindings).
  • openh264 — portable software fallback (the path that lets a Linux build ship without ffmpeg).
• decode::Decoder does the access-unit prep: avc1 (length-prefixed + out-of-band avcC) → Annex-B with SPS/PPS injected ahead of keyframes (reusing moq_mux::codec::{annexb,h264} helpers); avc3 passes through. Keyframe-gated.
• H.264 only (symmetric with what moq-video encodes); a non-H.264 rendition surfaces Error::UnsupportedCodec. Native decode of HEVC/VP9/AV1/VP8 (which ffmpeg did) is dropped.

rs/libmoq — new video-decode C API

• moq_consume_video_raw (+ _close / _frame / _frame_free), the video twin of moq_consume_audio_raw. Decoding happens inside the staticlib, so callers get a native decoder with no ffmpeg. cbindgen auto-emits the new types/functions into moq.h.

Public API changes

• rs/moq-video (additive): new pub mod decode with decode::{Consumer, Config, Kind, Frame}; new Error::{NoDecoder, UnsupportedCodec} variants (enum is #[non_exhaustive]). No existing signatures changed.
• rs/libmoq (additive C ABI): moq_consume_video_raw{,_close,_frame,_frame_free}, moq_video_frame, moq_video_decoder_output; new Error::Video variant (code -36).

Both are additive, so non-breaking, but this targets dev per the branch-targeting rules (touches rs/moq-video / rs/libmoq public surface).

Test plan

• decode round-trip unit tests (openh264 encode → decode) for both backends, incl. a macOS-gated VideoToolbox test.
• End-to-end libmoq test (video_raw_decode): publishes real H.264 and decodes it through the C API, asserting a 320×240 packed-I420 frame.
• cargo clippy clean, cargo fmt / taplo clean (via nix develop).
• cargo check --workspace --all-targets green.
• Runtime-verified on macOS (VideoToolbox + openh264). Windows/Linux-hardware decode backends are future follow-ups.

Follow-up (out of scope here)

• OBS plugin (Phase 2, after feat(obs): vendor the OBS plugin in-tree under cpp/obs #1787): rewrite cpp/obs/src/moq-source.cpp onto moq_consume_video_raw (hand OBS VIDEO_FORMAT_I420 directly, drop avcodec/swscale), strip ffmpeg from cpp/obs/CMakeLists.txt, re-enable a portable Linux binary in the obs-build matrix, and update doc/bin/obs.md.
• MediaFoundation / NVDEC / VAAPI decode backends (decode parity with encode).

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(Written by Claude)