feat(array): push struct validity into children

[miniex]

Summary

push_validity_into_children masks each field with the struct's top-level validity, so a row null at the struct level becomes null in every field ({a: 1, b: 2}, NULL -> {a: 1, b: 2}, {a: NULL, b: NULL}), mirroring Arrow's StructArray::flatten. remove_struct_validity drops the top-level validity to non-nullable; otherwise it is kept, and a struct with no top-level nulls is returned unchanged.

Each field is masked via a mask expression (per @gatesn's note on the issue, not the eager compute::mask of #5826). Open question: should this be a StructArray method, or a standalone mask expression in the new operator world?

Closes: #3859

Benchmark

For reference (not committed), vs hand-rolling the same masking without the fast path: with no top-level nulls the fast path is ~5-7x faster (0.26us vs 1.2us at 4 fields, 0.65us vs 4.5us at 16); with nulls the two are equal (~1.7us / ~6.3us), so the method adds no overhead.

Testing

cargo nextest run -p vortex-array passes (drops/preserves validity, intersecting field-level nulls, all-invalid, no-nulls fast path); fmt --all + clippy --all-targets --all-features clean.

---

I'm Korean, so sorry if any wording reads a little awkward.

[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

⚡ 4 improved benchmarks
❌ 4 regressed benchmarks
✅ 1581 untouched benchmarks
⏩ 4 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	copy_nullable[65536]	1 ms	1.4 ms	-24.28%
❌	Simulation	chunked_varbinview_into_canonical[(1000, 10)]	168.8 µs	205.5 µs	-17.84%
❌	Simulation	copy_non_nullable[65536]	908.3 µs	1,089.3 µs	-16.62%
❌	Simulation	compact_sliced[(4096, 90)]	750 ns	837.5 ns	-10.45%
⚡	Simulation	chunked_bool_canonical_into[(1000, 10)]	27.1 µs	16.6 µs	+63.22%
⚡	Simulation	chunked_varbinview_canonical_into[(100, 100)]	259.5 µs	224.4 µs	+15.66%
⚡	Simulation	chunked_varbinview_into_canonical[(100, 100)]	306.6 µs	271.2 µs	+13.02%
⚡	Simulation	rebuild_naive	109.3 µs	98.6 µs	+10.82%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing miniex:feat/push-struct-validity-into-children (2ecabe7) with develop (15cec3b)}

Footnotes

1. 4 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. ↩