perf(parquet/pqarrow): write FixedSizeBinary values directly

[fallintoplace]

Summary

• Writes Arrow FixedSizeBinary values directly from the fixed-width value buffer.
• Skips null slots with validity bit runs.
• Keeps stats, bloom filters, byte-stream-split, slices, nested lists, and dictionary paths covered.

Benchmark

Apple M1 Pro. 64K rows. 16-byte values. Median of 3 runs.

case	upstream main	this PR	change
required, stats off	1.77 ms, 5.84 MB	0.50 ms, 4.26 MB	-72% time, -27% bytes
required, stats on	2.28 ms, 5.85 MB	1.14 ms, 4.28 MB	-50% time, -27% bytes
nullable, stats off	1.92 ms, 5.95 MB	1.13 ms, 4.38 MB	-41% time, -26% bytes
nullable, stats on	2.42 ms, 5.95 MB	1.78 ms, 4.39 MB	-27% time, -26% bytes

Command:

go test ./parquet/pqarrow -run "^$" -bench "^BenchmarkWriteArrowFixedSizeBinary$" -benchmem -benchtime=2s -count=3

Tests

• PARQUET_TEST_DATA=/path/to/parquet-testing/data go test ./parquet/...
• PARQUET_TEST_DATA=/path/to/parquet-testing/data go test -race ./parquet/internal/encoding ./parquet/file ./parquet/metadata ./parquet/pqarrow
• go vet ./parquet/internal/encoding ./parquet/file ./parquet/metadata ./parquet/pqarrow