feat(moq-mux): synthesize fMP4/CMAF init for AV1 video

[kixelated]

Summary

moq-mux's fMP4/CMAF init-segment writer could only synthesize a sample entry for H.264 and H.265; every other codec fell through to UnsupportedSynthesis. Remuxing an AV1 MoQ broadcast to fMP4 therefore failed:

moq-cli subscribe --format fmp4   # (or accept ... --format fmp4) against an AV1 broadcast
Error: cmaf: can't synthesize CMAF init for video codec AV1(AV1 { profile: 0, level: 31, ... })

This adds the AV1 case so synthesize_video_trak builds an av01 sample entry carrying an av1C box.

Changes

• rs/moq-mux/src/codec/av1/mod.rs: new av1c_from_av1 helper, the inverse of the existing av1_from_av1c (import direction). Maps the catalog AV1 config (profile, level, tier, bit depth, chroma subsampling, monochrome, chroma sample position) into an mp4_atom::Av1c. Adds a bitdepth_flags helper (inverse of bitdepth) for the (twelve_bit, high_bitdepth) flag pair.
• rs/moq-mux/src/container/fmp4/mod.rs: synthesize_video_trak now handles VideoCodec::AV1, building an av01 sample entry + av1C.
• config_obus is left empty. moq-video publishes AV1 with the sequence header inline in the bitstream (the .av01 in-band case, analogous to hev1/avc3), so the decoder reads it from the keyframe rather than the out-of-band config record. The catalog's color fields (color primaries / transfer / matrix / full range) have no slot in av1C; they live in the sequence header OBU.
• doc/lib/rs/crate/moq-mux.md: brought the supported-codec list up to date (it also omitted VP8/VP9, which were already supported).

Public API

No public surface changes. av1c_from_av1 / bitdepth_flags are pub(crate); the new arm is inside a pub(crate) function. No wire or breaking change.

Branch targeting

Targets dev per CLAUDE.md: this is a catalog/container-format area change in moq-mux.

Test plan

• cargo test -p moq-mux (265 passed) — new av1_source_to_cmaf_export_synthesizes_av01 round-trips a synthesized init back to an av01 sample entry with a well-formed av1C (empty config_obus), plus bitdepth_flags_round_trip and av1c_round_trips_catalog_fields unit tests.
• cargo clippy -p moq-mux --all-targets -- -D warnings
• cargo fmt --all --check
• RUSTDOCFLAGS="-D warnings" cargo doc -p moq-mux --no-deps
• End-to-end moq-cli accept ... --format fmp4 + ffprobe: not run locally (this Windows box lacks pkg-config/ffmpeg for the full-workspace build); covered by the export round-trip test.

(Written by Claude)