Add unsigned writer compatibility fixture (#68)

[sehkone]

Summary

• Add pre-signing manifest and decoded archive compatibility fixtures for the fixed unsigned-writer input.
• Replace the self-comparison regression test with a public append_trailer test that checks the base, manifest, decoded archive, and stable footer fields.
• Keep compressed archive bytes and the compressor-dependent footer length out of the golden assertions.

Closes #68

Part of #66

Test plan

• Perturb the tar mtime and manifest field order separately; each fixture comparison fails with the affected block named.

• Run cargo test unsigned_writer_matches_pre_signing_block_fixtures to compare the public writer output with both fixtures.

• Run cargo fmt -- --check --config group_imports=StdExternalCrate.

• Run cargo clippy --all-targets -- -D warnings.

• Run cargo clippy --all-targets --features test-support -- -D warnings.

• Run cargo test.

• Run cargo test --features test-support.

[sehkone]

[Reviewer Round 1]

Approved — no findings.

The new test drives public append_trailer, pins manifest and decoded tar bytes to checked-in fixtures, and asserts the stable footer/base layout without goldening compressor-dependent data. It replaces the former self-comparison and satisfies the issue’s compatibility intent. PR linkage and test plan are also correct.

[sehkone]

[Review Verdict Round 1: APPROVED]