feat(java): expose manifest writer version

[everySympathy]

Summary

• expose the current manifest's writer library and semantic version metadata through Java
• use a typed WriterVersion result with optional prerelease and build metadata
• return Optional.empty() for older manifests that do not contain writer-version metadata

Compatibility coverage

DatasetTest exercises both compatibility directions through the public Dataset.getWriterVersion() JNI path:

• a historical v0.7.5 manifest without writer_version returns Optional.empty()
• the checked-in lance 2.0.0-beta.1 manifest verifies that library, core version, prerelease, and absent build metadata arrive in the correct Java fields

The prerelease fixture assertion catches dropped or swapped qualifier fields in the Rust-to-Java mapping.

Testing

• cd java && ./mvnw -Dtest=DatasetTest test (54 Java tests and 17 JNI Rust tests)
• cd java && cargo clippy --tests --manifest-path lance-jni/Cargo.toml -- -D warnings
• cd java && ./mvnw spotless:check

[lance-gatekeeper]

⚠️ Gate recommendation: approve with a non-blocking risk.

The Java/JNI mapping is a thin, faithful projection of the existing manifest contract. The non-blocking risk is regression coverage: the new test proves only a present library/core version, not legacy absence or prerelease/build qualifiers. Targeted fixture assertions would lock down the advertised boundary behavior.

[lance-gatekeeper]

The new API also promises Optional.empty() for legacy manifests and preserves prerelease/build qualifiers, but this coverage only exercises a newly written manifest’s present library/core version. Please add focused assertions for those boundary paths—existing legacy and prerelease fixtures can cover two of them—so JNI null/optional mapping regressions are caught.

[lance-gatekeeper]

Partially addressed on 942f3fd: the historical v0.7.5 assertion now covers the JNI null → Optional.empty() path. The qualifier test constructs WriterVersion directly, so it would still pass if JNI swapped or dropped prerelease / buildMetadata; qualifier transport remains uncovered. This non-blocking thread remains open.

[lance-gatekeeper]

Fixed on 7ae0f8e: the 2.0.0-beta.1 fixture now exercises prerelease presence and build-metadata absence through Dataset.getWriterVersion(). The focused suite passed 54 Java tests and 17 JNI Rust tests, closing the qualifier-mapping risk.

[lance-gatekeeper]

⚠️ Gate recommendation: approve with a non-blocking risk.

Legacy absence is now covered end to end. The remaining non-blocking risk is that prerelease/build qualifier transport still lacks an end-to-end Java/JNI assertion; exercising a qualifier-bearing manifest through Dataset.getWriterVersion() would close it.

[lance-gatekeeper]

✅ Gate recommendation: approve.

The qualifier-bearing historical fixture now exercises the remaining Java/JNI optional-field boundary, closing the prior regression-coverage risk while preserving the thin manifest projection.