Preserve unknown stsd codec bodies

[kixelated]

Fixes #194.

Summary

• preserve the body bytes of unknown stsd sample entries in Codec::Unknown
• encode unknown codecs as complete size/FourCC/body boxes
• retain valid box structure for atom kinds known elsewhere but unsupported as sample entries
• add a byte-for-byte dvh1 round-trip regression test

Root cause

Codec::decode discarded the body of unrecognized sample entries, and Codec::encode wrote only their FourCC. The FourCC was consequently interpreted as the next box's size field, corrupting the stsd structure during a decode/re-encode cycle.

Impact

In non-strict builds, unsupported sample entries now round-trip without losing their payload or corrupting the enclosing stsd. The public Codec::Unknown variant now carries both its FourCC and raw body bytes.

Validation

• cargo test --all-targets (237 tests passed)
• cargo check --all-targets --all-features
• cargo clippy --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check

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Review details

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 3637c9c5-1de5-41ce-bbb9-b21bbf673fc0

📥 Commits

Reviewing files that changed from the base of the PR and between ac798db and 153f611.

📒 Files selected for processing (2)

• src/moov/trak/mdia/minf/stbl/stsd/mod.rs
• tests/stsd.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/preserve-unknown-stsd-codecs

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[kixelated]

wtf is this syntax?

[bradh]

https://doc.rust-lang.org/book/ch19-03-pattern-syntax.html#using--bindings

I think the code is using unknown in two different ways, which probably means it needs a rewrite.