2: Add NullCount aggregate function

[gatesn]

Part of #7707.

Base: #7931
Next: #7930

Summary

• Add a NullCount aggregate wrapper and register it in the aggregate function session.
• Bridge Stat::NullCount through the Stat aggregate-function mapping introduced by Centralize aggregate stat bridge #7931.
• Add stats expression constructors used by the current pruning/stat expression path.

Checks

• cargo test -p vortex-array null_count
• cargo test -p vortex-array stat_expr
• ./scripts/public-api.sh
• cargo clippy --all-targets --all-features

[codspeed-hq]

Merging this PR will degrade performance by 16.64%

⚠️ 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.

❌ 6 regressed benchmarks
✅ 1210 untouched benchmarks
⏩ 24 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	new_bp_prim_test_between[i16, 32768]	121 µs	134.7 µs	-10.15%
❌	Simulation	new_bp_prim_test_between[i32, 16384]	95.3 µs	109.5 µs	-12.94%
❌	Simulation	new_bp_prim_test_between[i64, 32768]	178.8 µs	237.2 µs	-24.6%
❌	Simulation	new_bp_prim_test_between[i32, 32768]	141.7 µs	170.4 µs	-16.84%
❌	Simulation	new_bp_prim_test_between[i64, 16384]	115.9 µs	144.9 µs	-19.98%
❌	Simulation	new_alp_prim_test_between[f64, 16384]	127.7 µs	149.3 µs	-14.51%

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 ngates/stats-7707/null-count-stat-wrappers (85a04e0) with ngates/aggregatefn-stat-bridge-cleanup (7b76045)}

Footnotes

1. 24 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. ↩