fix: collect unreferenced data newer than retained manifests

[lance-gatefixer]

Summary

• List data files through the moving unverified-file age threshold during normal cleanup.
• Scan the complete managed data subtree when delete_unverified is enabled.
• Extend the failed-commit regression test for aged and explicitly unverified orphan files newer than every retained manifest.

Root cause

cleanup_old_versions capped the data subtree listing at the earliest retained manifest timestamp. Abandoned writes are not owned by any manifest and can be newer than that timestamp, so they never reached the existing age and delete_unverified checks.

Validation

• cargo test -p lance dataset::cleanup::tests (45 passed)
• cargo fmt --all
• cargo clippy --all --tests --benches -- -D warnings
• git diff --check

Fixes #8942

[lance-gatekeeper]

✅ Gate recommendation: approve.

The data listing now covers the union of manifest-proven stale files and files past the moving unverified threshold, while retained-manifest references and the existing recent-write guard continue to protect live data. This addresses the leak at the candidate-visibility boundary without changing the cleanup API or format.