Consolidate NodeRecordTask and test ordinal holes

[jaipilot]

What

• NodeRecordTask.java: extracted three blocks duplicated verbatim between callLegacy() and buildFullRecord() into shared private helpers: writeOmittedFeaturesAndEmptyNeighbors, checkNodeExists, writeNeighborSection.
• Grid.java: collapsed the useParallelConstruction if/else in builderWithSuppliers() (two branches differing only in which concrete Builder subtype is constructed) into a single ternary-selected variable, removing 3 duplicated statements.

Why

Both duplications were introduced by this PR's NodeRecordTask/ParallelGraphWriter rewrite, which had no dedicated unit test coverage anywhere in the repository. Extracting the duplicated logic reduces the surface that future edits to validation/hole/neighbor-writing semantics would otherwise have to keep in sync by hand across two call sites.

Behavior preservation

• Added testOrdinalHolesOnePhase/testOrdinalHolesTwoPhase to the existing TestRandomAccessOnDiskGraphIndexWriter (which already covered the batched vs. legacy paths for non-hole ordinals) to lock the OMITTED-ordinal branch touched by the cleanup, using an OrdinalMapper.MapMapper with gaps.
• Ran the same 6-test focused command against the unmodified PR-head production code (baseline) and the final candidate (candidate): identical pass/fail outcome (6/6 passed both times).
• Ran the full 5-module reactor build+test suite against the candidate: BUILD SUCCESS, 204/204 tests passed (2 pre-existing skips).
• Measured NodeRecordTask write timings (batched and legacy paths) before/after the extract-method refactor across 5 iterations each: medians differ by <3ms (noise level) -- no performance regression.

Limitations

• No JMH/PIT/architecture tooling is configured in this repository; evidence is limited to JUnit test execution, full reactor build status, and a manual timing comparison.
• Grid.java's builderWithSuppliers change was verified by full-module compile and the existing (unrelated) jvector-examples test suite; there is no dedicated unit test for this example/benchmark helper in the repository, before or after this change.

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