Validate typed atom kinds and preserve dref entries

[kixelated]

Fixes #183

Summary

• validate typed Decode, ReadFrom, and AsyncReadFrom paths against the atom FourCC before decoding the body
• replace Dref.urls with ordered Dref.entries supporting url , urn , and opaque unknown entries
• preserve unknown data-reference payloads and FourCCs during round trips
• add regression coverage for mismatched typed decoding and mixed dref entries

Root cause

The generic typed convenience implementations decoded T::decode_body without first checking header.kind == T::KIND. Dref also decoded every child directly as Url, so valid urn entries and unknown children could be silently reinterpreted and re-encoded as url .

API impact

Dref { urls: Vec<Url> } becomes Dref { entries: Vec<DrefEntry> }. Existing URL entries can be constructed with Url::default().into().

Validation

• cargo test --all-features (239 unit tests and 3 doc tests)
• cargo clippy --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check
• git diff --check