Return errors for malformed metadata atoms

[kixelated]

Fixes #179.

What changed

• Decode the four-byte subs codec-specific parameters through the checked decoding path, returning an error for truncated input.
• Validate nested iref box sizes before slicing and constrain each reference decode to its declared box body.
• Return Error::MissingContent when encoding version 0 or mime infe entries without their required content type.
• Add regression tests for truncated subs data, invalid iref sizes, and both invalid infe encoder states.

Why

These paths previously trusted attacker-controlled lengths or unwrapped optional fields. Malformed media could therefore panic, underflow, or loop instead of returning a normal parse/encode error.

The iloc size validation and unsupported infe version 1 handling described in the issue were already fixed on main; this PR addresses the remaining cases.

Validation

• cargo test --all-features (244 tests passed)
• cargo clippy --all-targets --all-features -- -D warnings
• cargo fmt --all
• git diff --check

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📥 Commits

Reviewing files that changed from the base of the PR and between ac798db and 606780a.

📒 Files selected for processing (3)

• src/meta/iinf.rs
• src/meta/iref.rs
• src/moov/trak/mdia/minf/stbl/subs.rs

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/issue-179-panic-errors

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[bradh]

This would be better as separate changes.

[kixelated]

Split into three focused draft PRs as requested:

• Return errors for missing infe content types #218 — return errors for missing infe content types
• Validate nested iref reference sizes #219 — validate nested iref reference sizes
• Return an error for truncated subs codec parameters #220 — return an error for truncated subs codec parameters

Closing this combined PR in favor of those independent changes.