perf(arrow): build partition filter mask with BooleanBufferBuilder

[anoopj]

What changes are included in this PR?

The partition splitter built each group's selection mask as a throwaway vec![false; num_rows] and bit-packed it into a BooleanArray via .into(), allocating an n-byte Vec<bool> plus a full packing pass per group per batch on every partitioned write. Build the packed buffer directly with BooleanBufferBuilder, setting only the selected bits.

Are these changes tested?

Covered by existing tests

[Fokko]

Asked my LLM friend for a benchmark, and it is more than I expected:

Results

Micro-benchmark isolating exactly the changed code (building each group's selection mask), arrow 59.3, criterion, macOS. Each iteration builds masks for all groups in one batch (i.e. one full split):

┌──────────────────────────────┬───────────────┬──────────────────────────┬─────────┐
│ config (batch rows × groups) │ old Vec<bool> │ new BooleanBufferBuilder │ speedup │
├──────────────────────────────┼───────────────┼──────────────────────────┼─────────┤
│ 1024 × 1                     │ 1.47 µs       │ 1.07 µs                  │ 1.37×   │
├──────────────────────────────┼───────────────┼──────────────────────────┼─────────┤
│ 1024 × 10                    │ 3.65 µs       │ 0.69 µs                  │ 5.30×   │
├──────────────────────────────┼───────────────┼──────────────────────────┼─────────┤
│ 8192 × 1                     │ 11.28 µs      │ 8.34 µs                  │ 1.35×   │
├──────────────────────────────┼───────────────┼──────────────────────────┼─────────┤
│ 8192 × 10                    │ 24.12 µs      │ 3.37 µs                  │ 7.15×   │
├──────────────────────────────┼───────────────┼──────────────────────────┼─────────┤
│ 8192 × 100                   │ 266.8 µs      │ 7.67 µs                  │ 34.8×   │
├──────────────────────────────┼───────────────┼──────────────────────────┼─────────┤
│ 65536 × 1                    │ 89.3 µs       │ 69.2 µs                  │ 1.29×   │
├──────────────────────────────┼───────────────┼──────────────────────────┼─────────┤
│ 65536 × 10                   │ 192.8 µs      │ 24.7 µs                  │ 7.79×   │
├──────────────────────────────┼───────────────┼──────────────────────────┼─────────┤
│ 65536 × 100                  │ 2128.9 µs     │ 35.8 µs                  │ 59.5×   │
└──────────────────────────────┴───────────────┴──────────────────────────┴─────────┘

[laskoviymishka]

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