perf(arrow/array): compare optional validity bitmaps in bulk

[fallintoplace]

What

• Speed up Equal and ApproxEqual when one array has no validity bitmap.

Why

• Comparing an absent bitmap with a materialized bitmap currently checks validity one row at a time.

Implementation

• Use BitmapAllSet on the present bitmap, scanning aligned words and respecting its slice offset.
• Return as soon as an invalid bit is found instead of counting the full range.
• Preserve empty-buffer handling and bitmap checks when declared null counts are inconsistent.

Benchmark

Command: go test -run '^$' -bench '^BenchmarkArrayEqualValidityBitmap$' -benchmem ./arrow/array

Apple M1 Pro, Go 1.26.3, GOMAXPROCS=1. Medians of six alternating before/after runs, 300 ms per case. Each case compares 1,048,576 values with one absent bitmap and one materialized bitmap. Binary/string values are 8 bytes each.

Case	Before	After	Speedup
Boolean, Equal	8,806.5 µs	21.3 µs	414.4x
Boolean, ApproxEqual	4,643.2 µs	20.9 µs	221.9x
Int8, Equal	5,868.4 µs	58.9 µs	99.6x
Int64, Equal	7,586.4 µs	494.7 µs	15.3x
Binary, Equal	11,979.7 µs	1,658.2 µs	7.2x
String, Equal	8,959.1 µs	1,895.6 µs	4.7x
String, ApproxEqual	19,381.8 µs	11,087.5 µs	1.7x

No new allocations. The benchmark also covers 64-value arrays, unaligned slices, and controls with both bitmaps present or both absent.

Compatibility

• No data-format changes.
• Adds one reusable bitmap utility; null semantics and slice offsets remain covered by the regression tests.

Tests

• go test ./arrow/...
• go test -race ./arrow/array
• go test -tags noasm ./arrow/array
• go vet -composites=false ./arrow/array

[zeroshade]

Non-blocking malformed-input edge case: BitmapAllSet([]byte{0xff}, 64, 0) panics. With n == 0, both loops are skipped, but the word-scan still evaluates buf[8:8], which is out of bounds for a one-byte slice. This also changes equality for a zero-length hand-built ArrayData with an out-of-buffer offset from vacuously true to panic. Normal validated arrays cannot reach it. An early if n == 0 { return true } preserves the expected empty-range contract.