perf(arrow/array): bulk append fixed-width empty values

[fallintoplace]

What changed

AppendEmptyValues used to call AppendEmptyValue in a loop. For larger batches, each value repeated the reserve, bitmap, and storage work.

This change makes fixed-width empty appends bulk operations:

• reserve space once
• zero the appended value range in one operation
• set validity bits in bulk
• clear boolean value bits separately from the validity bitmap
• keep the existing single-value path for count 1

The numeric generated source and template are updated together. The covered builders are numeric, temporal, interval, float16, decimal, fixed-size binary, and boolean.

Correctness

• Empty values stay valid, including when the builder reuses storage.
• Reused numeric and boolean storage is explicitly tested.
• bufferBuilder.Advance now zeroes skipped bytes as documented.
• BooleanBuilder.Resize now preserves the requested logical length when truncating.
• Zero and negative counts remain no-ops.

Benchmark

AppendEmptyValues(1024) on an Apple M1 Pro. Three runs were used for each result.

Builder	Before	After	Allocs
Int32	5.15 us	1.50 us	11 -> 5
Int64	6.44 us	2.52 us	11 -> 5
Timestamp	6.50 us	3.22 us	12 -> 6
Boolean	4.04 us	0.30 us	7 -> 5
Fixed-size binary	10.1 us	5.69 us	16 -> 7

Small counts keep the single-value path, so the biggest improvement is for larger batches.

Tests

• go test ./arrow/array
• go vet ./arrow/array
• Added focused correctness tests and representative benchmarks