feat(java): expose referenced file metadata

[everySympathy]

Summary

• expose every Lance data file referenced by a fragment, including data-overlay files
• expose manifest base paths so Java callers can resolve base-relative physical URIs
• expose the canonical deletion-file relative path for exact metadata/accounting lookups

Motivation

Java storage-accounting and maintenance clients currently see only FragmentMetadata.getFiles(), which omits data-overlay files. They also cannot resolve non-default base IDs or construct deletion-file paths without duplicating Lance naming rules. Together, these gaps prevent complete physical-file enumeration without falling back to object-store prefix listing.

This PR exposes the existing Rust manifest metadata through Java/JNI. Existing FragmentMetadata constructors remain source compatible.

Validation

• cargo fmt --all -- --check
• cargo clippy --all --tests --benches -- -D warnings
• cd java && ./mvnw test -Dtest=FragmentTest#testFragmentCreate+testDeletionFileRelativePath,MultiBaseTest#testCreateMode
• Java: 3 passed; JNI: 18 passed

[lance-gatekeeper]

✅ Gate recommendation: approve.

The revision now preserves Rust’s physical-file naming contract and exercises canonical base-plus-overlay enumeration through JNI. Both earlier findings are fixed.