bench: bit-packed compare-constant baseline

[joseph-isaacs]

Summary

• Adds a divan benchmark bitpack_compare in vortex-fastlanes that compares an Operator::Eq / Operator::Lt against an out-of-range constant on a BitPackedData array vs. an explicit "decompress, then Arrow compare" baseline that materialises the unpacked PrimitiveArray first.
• The constant is chosen as 1 << BW, just past the packable range, so a future compare-constant kernel can recognise it and short-circuit. Today both arms decompress; this PR establishes a baseline for that follow-up to land against.
• Grid sized for fast runs: len ∈ {1024, 65536}, bit_width ∈ {4, 16}, Eq + Lt.

The follow-up optimization (out-of-range fast path on BitPacked, plus the bitpack_constant analytical encoder) is in #PR2-PLACEHOLDER, stacked on this branch. Splitting the bench out lets the speedup PR show concrete numbers against this measured baseline.

Test plan

• cargo check -p vortex-fastlanes --benches
• cargo bench -p vortex-fastlanes --bench bitpack_compare records the slow baseline numbers prior to the follow-up landing

🤖 Generated with Claude Code

[codspeed-hq]

Merging this PR will improve performance by 18%

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

⚡ 4 improved benchmarks
✅ 1217 untouched benchmarks
🆕 16 new benchmarks

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	chunked_varbinview_canonical_into[(1000, 10)]	197.9 µs	162 µs	+22.19%
⚡	Simulation	chunked_varbinview_into_canonical[(100, 100)]	358.4 µs	323.5 µs	+10.78%
⚡	Simulation	chunked_varbinview_into_canonical[(1000, 10)]	211.2 µs	175.8 µs	+20.11%
⚡	Simulation	chunked_varbinview_opt_canonical_into[(1000, 10)]	224.8 µs	188.6 µs	+19.23%
🆕	Simulation	baseline_eq[16, 1024]	N/A	64.1 µs	N/A
🆕	Simulation	baseline_lt[16, 1024]	N/A	64.4 µs	N/A
🆕	Simulation	baseline_lt[16, 65536]	N/A	274.5 µs	N/A
🆕	Simulation	baseline_lt[4, 1024]	N/A	64.1 µs	N/A
🆕	Simulation	baseline_eq[16, 65536]	N/A	259.4 µs	N/A
🆕	Simulation	baseline_lt[4, 65536]	N/A	251.9 µs	N/A
🆕	Simulation	fast_eq_out_of_range[16, 65536]	N/A	291.1 µs	N/A
🆕	Simulation	fast_eq_out_of_range[4, 1024]	N/A	67 µs	N/A
🆕	Simulation	baseline_eq[4, 1024]	N/A	63.1 µs	N/A
🆕	Simulation	baseline_eq[4, 65536]	N/A	237.9 µs	N/A
🆕	Simulation	fast_eq_out_of_range[16, 1024]	N/A	67.7 µs	N/A
🆕	Simulation	fast_lt_out_of_range[16, 65536]	N/A	306.3 µs	N/A
🆕	Simulation	fast_lt_out_of_range[4, 65536]	N/A	262.1 µs	N/A
🆕	Simulation	fast_eq_out_of_range[4, 65536]	N/A	246 µs	N/A
🆕	Simulation	fast_lt_out_of_range[16, 1024]	N/A	67.9 µs	N/A
🆕	Simulation	fast_lt_out_of_range[4, 1024]	N/A	87.8 µs	N/A

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing claude/bitpack-compare-bench-KGPS3 (5f53991) with develop (faf7e42)}