Add nullable UnionArray

[connortsui20]

Tracking issue: #8769

Implements the array portion of #8769 on top of #8791.

• adds the canonical sparse UnionArray
• stores outer union validity exclusively in the nullable u8 type IDs child
• treats child validity as inner/selected-value nullability, not outer validity
• ignores the type-ID payload at outer-null rows during validation
• preserves outer and inner nulls through scalar_at, serde, constants, chunking, filtering, slicing, masking, recursive canonicalization, and compression
• recursively builds valid defaults for unselected children and returns an error for genuinely uninhabited schemas
• canonicalizes empty unions without inventing placeholder values
• supports non-nullable-index take; nullable-index take remains an explicit unsupported error for now

[codspeed-hq]

Merging this PR will not alter performance

✅ 1670 untouched benchmarks
⏩ 52 skipped benchmarks¹

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_{Comparing agent/union-array-nullability (40479b3) with agent/union-scalar-nullability (d5458d2)}

Footnotes

1. 52 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩