perf(parquet/pqarrow): build nullable fixed-size list children directly

[fallintoplace]

Summary

• keep the existing zero-null fixed-size-list path unchanged
• validate nullable parent spans once
• use the existing slice concatenation for a small number of validity runs
• build one child array with a typed Take for highly fragmented validity
• avoid one temporary Arrow array per validity run

Why

Parquet does not write child values for null fixed-size-list parents. Arrow still needs list_size child slots for every parent, so the reader has to add null child values.

The old nullable path creates one temporary array for each validity run and concatenates them. Alternating validity can therefore create one temporary array per parent row.

Benchmark

Apple M1 Pro, 65,536 FixedSizeList<int32, 4> parents:

validity	old	new	old allocs	new allocs
10% nulls	~4.4 ms	~1.3 ms	85,220	46
alternating nulls	~23–30 ms	~1.1 ms	458,770	46
one clustered null run	~0.3 ms	~0.3 ms	28	27

The low-run path is kept for the clustered case, where creating a single pair of slices is cheaper than building an index array.

Tests

• go vet ./parquet/pqarrow
• go test -race ./parquet/pqarrow -run 'TestBuildFixedSizeListArray|TestParquetArrowIO/TestFixedSizeList'
• PARQUET_TEST_DATA=parquet-testing/data ARROW_TEST_DATA=arrow-testing/data go test ./...