perf(array): accumulate nested builder validity without a null buffer [builders-child-stack]

[robert3005]

Rationale for this change

Stacked on #8964 — review that first.

After the parent merges the arrays are lazy but validity is not. With this change the validity is lazy as well

[codspeed-hq]

Merging this PR will not alter performance

✅ 1842 untouched benchmarks
⏩ 55 skipped benchmarks¹

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_{Comparing claude/builders-lazy-validity-9ze0t6 (dd4420b) with claude/builders-canonical-children-9ze0t6 (c9bacae)}

Footnotes

1. 55 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. ↩

[robert3005]

Do we still need LazyBitBufferBuilder?

[robert3005]

On LazyBitBufferBuilder: yes, it is still carrying its weight, in two roles.

1. It is ValidityBuilder::pending — the bits that arrive one row at a time from append_scalar / append_n_nulls. Its laziness (no buffer at all until the first null) is exactly what makes an all-valid builder cost nothing, so ValidityBuilder would have to reimplement it to drop it.
2. The flat builders use it directly: BoolBuilder, PrimitiveBuilder, DecimalBuilder, VarBinViewBuilder, plus varbinview::compute::zip. Those need bit-level operations a run cannot express — set_bit at an arbitrary index (PrimitiveBuilder's uninit ranges) and append_buffer.

What this PR does remove is the nested builders' use of it: ListBuilder, ListViewBuilder, StructBuilder and FixedSizeListBuilder all hold a ValidityBuilder now, and nothing outside ValidityBuilder builds a bit buffer for a nested builder.

I did not migrate the flat builders on purpose: a flat builder copies its values into one contiguous buffer, so there is no chunk boundary for validity runs to line up with, and keeping runs there would hand downstream a chunked bool array for an otherwise flat array. Say the word if you would rather have one accumulator everywhere and I will do it.

[robert3005]

Collapsed into #9064 along with #8964 and #9131.