fix: let archived fixtures bypass Git version gates (#1622)

[Sebastian Thiel (Byron)]

Tasks

This section is for Byron only. Models continuing this PR must not add, remove, check, uncheck, rename, or reorder checkboxes here.

• refackiew

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Everything below this line was generated by Codex GPT-5.

Created by Codex on behalf of Byron. Byron will review before this is ready to merge.

Summary

• add Git-version-aware scripted fixture loaders that require an identity-matching archive when the installed Git is incompatible
• keep required fixtures in a dedicated cache, honor archives even with GIX_TEST_IGNORE_ARCHIVES, and never generate them with incompatible Git
• run the affected rev-spec tests against the Git 2.50.1 archive on Git 2.47.x, while skipping only when the matching SHA-1/SHA-256 archive is unavailable
• migrate Git >= 2.31 worktree tests and restore 32-bit CI to Debian stable

Git 57fb139b5e introduced reversed :/ traversal in 2.47.x; Git 0ff919e87a restored youngest-first ordering in 2.48.0.

Refs #1622

Validation

• cargo test -p gix-testtools --all-features --lib
• cargo test -p gix-testtools --no-default-features --lib
• cargo nextest run -p gix revision::spec::from_bytes --no-default-features --features max-performance-safe,comfort,basic
• GIX_TEST_FIXTURE_HASH=sha256 cargo nextest run -p gix revision::spec::from_bytes::regex::find_youngest_matching_commit --no-default-features --features max-performance-safe,comfort,basic
• focused worktree tests
• cargo clippy -p gix-testtools --all-targets --all-features -- -D warnings
• cargo clippy -p gix --tests -- -D warnings
• cargo doc -p gix-testtools --no-deps
• cargo fmt --all -- --check
• git diff --check
• Codex commit review, including a simulated Git 2.47 archive-path test