esds: fix descriptor size varint and AudioSpecificConfig bit parsing

[kixelated]

Fixes the concrete defects catalogued in #192 (all in src/moov/trak/mdia/minf/stbl/stsd/mp4a/esds.rs).

Changes

1. Descriptor size varint (encode). The base-128 length was emitted least-significant group first, so a 200-byte body wrote C8 01 (decoded as 9217) and a zero-length body wrote no length byte at all. It now emits most-significant group first with the continuation bit on all but the last byte — matching the decoder — and always writes at least one byte.

2. Size varint (decode). A size field whose 4th byte still has the continuation bit set now returns Error::InvalidSize instead of silently truncating and desyncing the parse.

3. AudioSpecificConfig. Replaced the ad-hoc byte masking with an MSB-first bit reader/writer, fixing:

• extended audioObjectType (≥ 32, e.g. xHE-AAC = 42) — the 6-bit extension was OR'd on top of itself (missing << 3);
• extended-profile channelConfiguration extraction;
• the samplingFrequencyIndex == 15 escape — the explicit 24-bit sample rate was misread, discarded, and unencodable. It's now stored in a new sample_rate field and round-trips.
• encoding an unrepresentable object type now errors instead of truncating.

4. ES_Descriptor flags byte. streamDependenceFlag / URL_Flag / OCRstreamFlag were read and discarded, so when set the following dependsOn_ES_ID / URL / OCR_ES_ID bytes were misparsed as child descriptors. They're now parsed and preserved via new depends_on_es_id / url / ocr_es_id / stream_priority fields.

Tests

Added a tests module covering: varint round-trips (incl. empty payload), the 81 48 (200-byte) and 4-byte-desync regressions, basic/extended/explicit-rate AudioSpecificConfig round-trips, and the ES flags round-trip. Full suite (244 tests + doc-tests), clippy -D warnings, and fmt --check all pass.

⚠️ Breaking change

DecoderSpecific and EsDescriptor gain public fields, and EsDescriptor is no longer Copy. Existing struct literals need ..Default::default() (all in-tree call sites are updated).

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📥 Commits

Reviewing files that changed from the base of the PR and between ac798db and 3db4afe.

📒 Files selected for processing (5)

• src/moov/trak/mdia/minf/stbl/stsd/mp4a/esds.rs
• src/moov/trak/mdia/minf/stbl/stsd/mp4a/mod.rs
• src/test/bbb.rs
• src/test/esds.rs
• src/test/h264.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/esds-192

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