test(writer): mutation-cover VortexWriter global-dict + factory paths (74% → 89%)

[dfa1]

Adds VortexWriter to the writer pitest scope and closes the bulk of its gaps — chiefly the global-dictionary feature, which was barely tested end to end (74% → 89% killed, strength 92%).

What

• VortexWriterDictDecisionTest — unit-tests the pure dict-decision helpers directly (made package-private): isDictCandidate / isUtf8DictCandidate (cardinality + 50%-ratio gates, empty, single-value, F16/F32 exclusion), codePTypeForSize (U8/U16/U32 width boundaries), primitiveArrayLen, readPrimitiveElement. These choices only affect encoding, not the decoded values, so a round-trip can't pin them.
• GlobalDictPrimitiveTest — round-trips low-cardinality I32/I64/F64 columns through the global dict build (the integer/float counterpart to the existing GlobalDictUtf8Test), plus high-cardinality and single-value fallbacks and the globalDict-disabled opt-out.
• VortexWriterTest — covers the create(WriteRegistry) factory overload.

Finding (real bug)

Mutation coverage surfaced a write/read incompatibility: the writer's global dict admits I8/I16 columns, but the reader's lazy dict decode only supports I32/I64/F64 — read-back throws "unsupported ptype for lazy dict: I16". Pinned by lowCardinality_i16_globalDict_readerRejects. The fix belongs to the reader's dict decode; flagging it here.

Remaining survivors (29)

Honest accounting — not chased in this PR:

• Structural (~14): segment 64-byte alignment math, layout / Array-FlatBuffer build, dict-flush internals — killable only with file-structure/size assertions.
• Impractical (~2): the U32 code path needs a 65k+-distinct dictionary.
• Unreachable/equivalent (~3): the F32 case in the dict build (F32 is excluded from dict), the I8 build path (reader gap above).

Result

VortexWriter 74% → 89%; the global-dict decision layer is now fully and directly pinned.

Verify

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

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