feat(java): count indexed rows on explicit scalar segments

[hfutatzhanghb]

feat(java): count indexed rows on explicit scalar segments

Summary

• Pin the existing Dataset.countIndexedRows method to the named scalar index, and reject filters that cannot be planned against that index. The three-argument signature is unchanged. This corrects the method so it counts with the index it was given, rather than scanning or selecting another index.
• Add an overload that opens only the requested physical segment UUIDs. Their current fragment coverage defines the count scope, and an explicit fragment list must match that coverage. A selection that omits a fragment-reuse contributor for that scope is rejected; the error names the missing UUIDs.
• Keep validation and count semantics in the Rust core. Deleted rows stay excluded, including on datasets that use stable row IDs.
• The three-argument signature is unchanged and the segment overload is additive. This does not change a published method signature, so it should not be labeled as a public API break.

Test plan

• ./mvnw spotless:apply and ./mvnw spotless:check in java/
• ScalarIndexTest covers independent segment counts, A+B versus the full logical index, coverage mismatch, unknown and wrong-index UUIDs, duplicate and empty input, deletions, stable row IDs, and proof that an unselected segment file is not opened
• Local Cargo compile and tests were not run on this machine. Remote CI is the compile and test gate.
• Confirm existing three-argument Java callers still compile

[claude]

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[hfutatzhanghb]

Hi, @Xuanwo @wjones127 @BubbleCal Could you please help review this PR when have free time? Thanks very much!

[lance-gatekeeper]

✅ Gate recommendation: approve.

Named-index counts now respect UUID selection on both modern and legacy storage, while retaining deletion filtering and the complete-contributor requirement after fragment reuse. Both earlier findings remain fixed.

The public Rust open_scalar_index_segments signature gains a parameter; the Java overload is additive. Please mark this PR with the breaking-change label.