mp4v: add sample entry

[bradh]

From ISO/IEC 14496-14:2020 Section 6.7.

@kixelated This re-uses the esds (ESDBox) that is currently under mp4a. Thoughts on possible re-arrangement of the files / implementation?

[coderabbitai]

Walkthrough

The PR adds Mp4v (MPEG-4 video) support: a new Any::Mp4v variant, a mp4v module and public re-export, a Codec::Mp4v variant with Decode/Encode handling, and a new public Mp4v atom struct (fields: visual, esds, btrt, taic) implementing Atom::KIND, decode_body, and encode_body. A unit test verifies encode/decode roundtrip.

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 20.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'mp4v: add sample entry' directly and clearly summarizes the main change—adding Mp4v support to the codebase.
Description check	✅ Passed	The description is directly related to the changeset, citing ISO/IEC 14496-14:2020 Section 6.7 and explaining the reuse of esds implementation.

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✨ Finishing touches

• 📝 Generate docstrings

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[kixelated]

If you want to move esds, go for it. I don't really understand it well enough.

[bradh]

I'm not sure that the existing esds implementation is enough for the visual case. #76 tracks the bit fiddling we need for the full audio case, and visual looks even more confusing.

So I'm going to park this for now.