Add vortex.is_nan expression with stats-based pruning

[Ecthlion]

Summary

• Tracking Issue: Expressions missing that could improve pushdown #9913

Adds the IS NAN expression called out in #9913, implemented as a strict RowFn as suggested in the issue: null inputs propagate to null outputs (output nullability follows the input), and only primitive float inputs are accepted.

Changes

• New vortex.is_nan scalar function in vortex-array implemented with the RowFn row-wise kernel framework, registered in the scalar fn session and exposed as expr::is_nan / expr::bound::is_nan.
• Stats rewrites mirroring the is_null rules so the predicate prunes from zone maps:
  • falsify: nan_count(x) = 0 or all_non_nan(x)
  • satisfy: nan_count(x) = row_count() or all_nan(x)
• Unit tests for evaluation (valid/null floats, constants, sliced arrays, non-float rejection), dtype strictness, display, and the falsify/satisfy rewrites.

AI disclosure: this change was developed with assistance from Kimi K3.

[codspeed]

Merging this PR will degrade performance by 16.65%

⚠️ 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
❌ 5 regressed benchmarks
✅ 2247 untouched benchmarks
⏩ 218 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	WallTime	arrow_checked_add_u32_neon[16384]	12.3 µs	20.3 µs	-39.28%
❌	WallTime	mul_u64_nonnull_neon	15.5 µs	21.3 µs	-27.39%
❌	WallTime	mul_i64_nonnull_neon	17.3 µs	20.4 µs	-14.82%
❌	WallTime	multiply_shapes_neon[(16384, PerRowPerRow)]	17.4 µs	20.4 µs	-14.7%
❌	WallTime	words_gather_scalar_avx2[65536]	8.2 µs	9.4 µs	-11.93%
⚡	WallTime	filtered_sink_i64_avx512[OneNullInEight]	26.6 µs	22.4 µs	+18.88%

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 Ecthlion:add-is-nan-expression (09f5aff) with develop (f5b3b26)}

Footnotes

1. 218 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩