perf(arrow/array): bulk append fixed-size list nulls and empty values

[fallintoplace]

What does this PR do?

• reserves fixed-size list parent slots once in AppendNulls and AppendEmptyValues
• updates the parent validity bitmap in bulk
• appends all child null or empty values with one bulk builder call

The child append count is listSize * n, so the parent and child lengths stay in sync.

Why?

Both methods currently loop over every parent value. Each parent append also loops over every child value.

This removes the parent loop and lets child builders use their bulk append paths. The change is independent, but it also composes with #1173 and #1180.

Benchmarks

Apple M1 Pro, 65,536 FixedSizeList<Int32> rows, GOMAXPROCS=1.

Standalone on main:

list width	nulls before	nulls after	change	empty before	empty after	change
4	1.10 ms	0.86 ms	-22%	1.55 ms	1.07 ms	-31%
16	3.76 ms	2.74 ms	-27%	5.43 ms	3.90 ms	-28%
64	13.03 ms	11.07 ms	-15%	19.35 ms	14.86 ms	-23%

With #1173 and #1180 applied underneath:

list width	nulls before	nulls after	change	empty before	empty after	change
4	1.56 ms	0.18 ms	-88%	2.01 ms	0.20 ms	-90%
16	4.69 ms	0.33 ms	-93%	6.89 ms	0.42 ms	-94%
64	16.27 ms	0.74 ms	-95%	22.18 ms	0.92 ms	-96%

GOMAXPROCS=1 go test ./arrow/array -run '^$' -bench '^BenchmarkFixedSizeListBuilderBulkAppend$/(nulls|empty)/rows=65536/width=(4|16|64)$' -benchmem -benchtime=300ms -count=5

Tests

• added coverage for unaligned parent validity bits
• verifies parent validity and null counts
• verifies child length, null slots, and zero-valued empty slots
• ran go test ./arrow/...
• ran go test -race ./arrow/array
• ran go vet ./arrow/array

[zeroshade]

Parent/child length synchronization is right in both paths, and delegating to the child builders' bulk appends is the correct layering — this composes cleanly with #1173/#1180. The nested ValidateFull test and slot-boundary assertions cover the tricky cases.

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This review was drafted with the help of an AI-assisted tool and may contain mistakes; it was reviewed and confirmed by an Apache Arrow maintainer before posting. See the contributing guide for what the project considers a maintainer review.