T6.1: AV1 encoder/decoder with HW probe + SVT-AV1 SW fallback

- New: av1_obu.rs — OBU framer, depacketizer, keyframe detection, LEB128 helpers
- New: dav1d.rs — SW AV1 decoder wrapper (shiguredo_dav1d)
- New: svt_av1.rs — SW AV1 encoder wrapper (shiguredo_svt_av1)
- Add CodecId::Av1Main = 12 with match-arm fixes in downstream crates
- Add VideoToolboxAv1Decoder for macOS M3+ HW decode
- Add MediaCodecAv1Encoder/Decoder for Android (video/av01)
- Add extract_sequence_header_obu() helper for AV1 decoder CSD
- Add 10-frame encode-decode roundtrip test (svt_av1 + dav1d)
- Fix clippy unused import in dav1d.rs
- 15 tests; all workspace tests pass; cargo fmt clean

crates/wzp-relay/src/conformance.rs

@@ -236,7 +236,7 @@ pub fn payload_size_bound(codec: CodecId) -> usize {
         CodecId::Codec2_3200 => 30,
         CodecId::Codec2_1200 => 30,
         CodecId::ComfortNoise => 16,
-        CodecId::H264Baseline | CodecId::H265Main => 1400,
+        CodecId::H264Baseline | CodecId::H265Main | CodecId::Av1Main => 1400,
     }
 }