Robust CMAF Ingestion Pipeline

[google-labs-jules]

This change enhances the fMP4 ingestion pipeline to robustly handle fragmented MP4 (CMAF) streams.

Key improvements:

• Introduced AtomReader in rs/hang/src/import/atom_reader.rs to manage a BytesMut buffer, allowing correct parsing of atoms split across network chunks (partial reads).
• Implemented zero-copy extraction of mdat payloads using Bytes::slice / split_to, reducing memory pressure during high-bitrate ingestion.
• Enforced strict CMAF atom ordering checks (e.g., styp/prft before moof, mdat after moof) with warning logs for violations.
• Refactored Fmp4 in rs/hang/src/import/fmp4.rs to use AtomReader and handle the parsing loop efficiently.
• Added comprehensive unit tests for AtomReader covering partial headers, partial bodies, and multiple atoms.

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PR created automatically by Jules for task 1425664028654606659 started by @nicolaslevy

[google-labs-jules]

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