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

Cargo.lock

@@ -5204,6 +5204,44 @@ dependencies = [
  "windows-sys 0.60.2",
 ]
 
+[[package]]
+name = "shiguredo_cmake"
+version = "4.3.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b2c6a73b295ff44900705fa5fdede6f6d425017964ad1ed8368376bcc83d85fa"
+dependencies = [
+ "cmake",
+ "shiguredo_toml",
+]
+
+[[package]]
+name = "shiguredo_dav1d"
+version = "2026.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bc6287d7cc9b0110e8c316642cf53434bd15220510c325e1e1235a41d3cf7f60"
+dependencies = [
+ "bindgen",
+ "shiguredo_toml",
+]
+
+[[package]]
+name = "shiguredo_svt_av1"
+version = "2026.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9c8dc78e51f59744f3f545b94306761cd2d75616012cb10426135aadb01e466a"
+dependencies = [
+ "bindgen",
+ "log",
+ "shiguredo_cmake",
+ "shiguredo_toml",
+]
+
+[[package]]
+name = "shiguredo_toml"
+version = "2026.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b475c218cf15d056ed3e48c9e98693135b261b79530881e63b162650b236fdec"
+
 [[package]]
 name = "shiguredo_video_toolbox"
 version = "2026.1.1"
@@ -7927,6 +7965,8 @@ dependencies = [
  "bytes",
  "ndk 0.9.0",
  "rand 0.8.6",
+ "shiguredo_dav1d",
+ "shiguredo_svt_av1",
  "shiguredo_video_toolbox",
  "tracing",
  "wzp-proto",