perf(cleanup): avoid reading manifests for version retention

[ziwenzhang]

Summary

• Optimize CleanupPolicyBuilder::retain_n_versions to use Dataset::version_refs() instead of Dataset::versions(), avoiding reading and deserializing every manifest when only version numbers are needed.
• Add a focused regression test that verifies the retention threshold remains correct and that retain_n_versions
  performs zero object-store reads.

Testing

• cargo test -p lance retain_n_versions_does_not_read_manifests -- --nocapture
• cargo test -p lance cleanup_and_retain_3_recent_versions -- --nocapture
• cargo fmt --all -- --check
• cargo clippy -p lance --lib --tests --benches -- -D warnings

[ziwenzhang]

cc @zhangyue19921010 would you mind taking a look when you have a chance? Thanks!

[lance-gatekeeper]

✅ Gate recommendation: approve.

The retention cutoff depends only on ordered attached version IDs, so reusing the lightweight version-reference listing preserves the existing branch, gap, and detached-manifest semantics while removing per-manifest payload reads. The focused regression covers both the cutoff and that I/O contract.