test(writer): kill behavioral VortexWriter survivors; parameterize dict-decision tests

[dfa1]

Follow-up to #99/#100. Closes the remaining behavioral VortexWriter mutation survivors and tidies the dict-decision tests.

Kills

• isUtf8DictCandidate cardinality-at-MAX — seen.size() > MAX must not trip at exactly MAX (boundary).
• rowCountForValidation on a Collection column — a two-column chunk (List<LocalDate> extension + same-length long[]) is accepted, pinning that the collection is measured by element count, not a constant.

Cleanup

VortexWriterDictDecisionTest is now @ParameterizedTest with proper Given/When/Then — the case params are the Given, result = helper(...) the When, the assertion the Then.

Remaining survivors (~30, not chased — by design)

Honest accounting: these are not correctness gaps:

• Encoding-choice / round-trip-identical — dict-vs-fallback selection, the frequency List.sort, the zstd codec-list toggle: all produce different-but-valid files that decode to the same values.
• 64-byte segment alignment math — a real Arrow contract, but self-consistent offsets still round-trip; killing needs internal segment-offset assertions.
• Dead-by-exclusion — the I8/I16/F32 arms of the dict-build switches (those ptypes are excluded from dict by fix(writer): exclude I8/I16 from global dict — reader can't decode narrow-int dict #100).
• Impractical — the U32 code path needs a 65k+-distinct dictionary.

VortexWriter sits at 89% killed (strength 92%); the global-dict decision layer and all behavioral paths are pinned.

Verify

./mvnw -pl writer -am -P pitest verify

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