feat: AV1 OBU parsing, sequence header, and codecs parameter string

[tobbee]

Summary

Extends the basic AV1 support (previously just the opaque av1C box) with the elementary-stream understanding needed for AV1 streaming in HLS/DASH/CMAF/MoQ manifests. This lays the foundation (codec string + OBU/sequence-header parsing); DRM protect-ranges and key-frame detection are natural follow-ups that build on the OBU layer added here.

What's included

OBU layer (av1/obu.go) — the AV1 analog of NALU handling:

• OBUType with all defined types, ParseOBUHeader (incl. the extension header), ReadLEB128, and SplitOBUs to split an av1C configOBUs field, a coded sample, or a temporal unit into OBUs. AV1 carries no emulation-prevention bytes, so parsing reads the raw bytes directly.

Sequence header + codec string (av1/seqhdr.go, av1/mime.go):

• ParseSequenceHeader / SequenceHeader — a faithful walk of sequence_header_obu() (spec §5.5), including the offset-critical operating-points loop and full color_config(), exposing profile/level/tier, Width()/Height(), bit depth, and color configuration.
• CodecConfRec.SequenceHeader() — parse the sequence header carried in an av1C record.
• RFC 6381 codecs parameter per the AV1-ISOBMFF binding: CodecConfRec.CodecString (mandatory av01.P.LLT.DD) and SequenceHeader.CodecString (full form with the color suffix, collapsing to the short form only when all six color fields are at their defaults).

Integration:

• av1C box Info() now prints the codecs parameter, coded resolution, and color configuration.
• mp4ff-pslister parses the av1C record of av01 tracks in mp4 files and prints the sequence header + codecs parameter.

Testing

• Unit tests for OBU header/LEB128/splitting, sequence-header parsing, and codec-string formatting (tier, 10/12-bit, monochrome, 4:4:4, default collapse).
• Validated against all 11 AOM fate-suite AV1 vectors (opt-in via MP4FF_AV1_TESTVECTORS_DIR): every temporal unit splits with full byte consumption, and every parsed resolution matches the IVF display header — including the SVC files (multiple operating points, extension flags) and dynamic-resolution streams.
• Fuzz targets FuzzSplitOBUs / FuzzParseSequenceHeader (no panics over millions of executions).
• New golden tests in mp4ff-pslister against a small real av01 init segment.

go test ./..., go vet, and golangci-lint all clean.

🤖 Generated with Claude Code