perf(arrow/array): remove map validity slice

[fallintoplace]

Summary

• Replace the temporary all-true validity slice in MapBuilder.adjustStructBuilderLen.
• Reuse the existing all-valid validity bitmap fast path.
• Add a benchmark for NewMapArray after bulk key/item appends.

The benchmark stops the timer while setting up the map entries, so it focuses on the finalization path where the temporary slice was created.

Benchmark

Measured on Apple M1 Pro, macOS arm64, Go 1.26.3. Median of 3 runs. Each result is time/op, B/op, and allocs/op.

Bulk entries	Before	After
32	6.59 us, 4,392 B, 54 allocs	6.20 us, 4,392 B, 54 allocs
1,024	9.00 us, 5,608 B, 55 allocs	7.69 us, 4,584 B, 54 allocs
65,536	92.9 us, 88.2 KB, 55 allocs	31.0 us, 22.7 KB, 54 allocs

For 65,536 entries, this reduces bytes allocated by about 74% and removes one allocation per operation. The 1,024-entry case removes the temporary 1,024-byte slice as well.

Command:

go test -vet=off ./arrow/array -run '^$' -bench '^BenchmarkMapBuilderNewArrayAfterBulkChildren$' -benchmem -benchtime=500ms -count=3 -cpu=1

Tests

• go test ./arrow/array -run 'TestMap' -count=1
• go test ./... -count=1 with the pinned arrow-testing and parquet-testing submodules initialized

[zeroshade]

Verified the map validity fast path preserves output semantics and raw buffers across bitmap boundaries, builder reuse, mixed builder workflows, nested maps, invalid-state handling, and randomized cases. Array, race, vet, LSP, allocator, and performance checks pass. The allocation reduction is reproducible.

Nice improvement—thanks!