perf: branchless primitive zip kernel

[joseph-isaacs]

Summary

Adds a dedicated primitive zip kernel that selects values branchlessly per row.

The generic zip path copies runs of if_true/if_false between mask boundaries — fast for clustered masks but degrading to per-element work on fragmented masks. This kernel walks the mask as 64-bit chunks and blends both sides per row with no data-dependent branch, so the inner loop stays branch-free and auto-vectorizable regardless of mask shape. Result validity reuses the shared zip_validity helper, which expresses validity selection as a (lazy) zip over the two validity bitmaps.

Stacked on #8275 (branchless boolean zip kernel). The shared zip_validity produces a boolean zip over the validity bitmaps, so the nullable path is only fast once the bool kernel lands — this PR is based on that branch and should merge after it. The diff here is primitive-only.

Changes

• vortex-array/src/arrays/primitive/compute/zip.rs: branchless per-row value blend; validity via the shared zip_validity; tests spanning the 64-bit chunk boundary + remainder, non-nullable and nullable.
• vortex-array/src/arrays/primitive/compute/mod.rs, .../vtable/kernel.rs: register the kernel.
• vortex-array/benches/primitive_zip.rs: a small divan bench — one Fragmented (alternating) mask, non-nullable and nullable.

Performance (divan, local walltime, median)

At LEN = 65_536 (matching the original measurements), the nullable Fragmented case — which routes validity through the shared zip_validity → boolean zip — drops from ~43 ms (generic builder, before the bool kernel) to ~73 µs; non-nullable is ~58 µs. The committed bench uses LEN = 16_384 so each case stays well under a few hundred microseconds under CodSpeed's instruction-count simulation.

Testing

• vortex-array zip tests pass (24, incl. the new primitive cases); cargo +nightly fmt, clippy -D warnings (default + all-features), and cargo doc -D warnings clean.

https://claude.ai/code/session_01N5ivPiCJy7dGQjMEP7ips9

[codspeed-hq]

Merging this PR will not alter performance

⚠️ 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.

⚡ 1 improved benchmark
❌ 4 regressed benchmarks
✅ 1510 untouched benchmarks
🆕 2 new benchmarks

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	bitwise_not_vortex_buffer_mut[128]	216.9 ns	275.3 ns	-21.19%
❌	Simulation	bitwise_not_vortex_buffer_mut[1024]	278.6 ns	336.9 ns	-17.31%
❌	Simulation	bitwise_not_vortex_buffer_mut[2048]	342.2 ns	400.6 ns	-14.56%
❌	Simulation	chunked_varbinview_canonical_into[(100, 100)]	274.6 µs	309.3 µs	-11.25%
⚡	Simulation	chunked_varbinview_canonical_into[(1000, 10)]	198.2 µs	161.9 µs	+22.47%
🆕	Simulation	nonnull	N/A	252.5 µs	N/A
🆕	Simulation	nullable	N/A	285.2 µs	N/A

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing claude/primitive-branchless-zip (bdc1c4b) with claude/bool-branchless-zip (7ad4b18)}