Add NullCount aggregate function

[gatesn]

Part of #7707.

Replacement for #7929 after the accidental merge was reverted before #7931 landed.

Base: develop
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 17.06%

⚠️ 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.

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

❌ 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[i32, 32768]	141.2 µs	170.4 µs	-17.14%
❌	Simulation	new_bp_prim_test_between[i64, 16384]	115.2 µs	144.9 µs	-20.48%
❌	Simulation	new_bp_prim_test_between[i64, 32768]	178.1 µs	237.2 µs	-24.9%
❌	Simulation	new_alp_prim_test_between[f64, 16384]	126.9 µs	149.3 µs	-15.04%
❌	Simulation	new_bp_prim_test_between[i16, 32768]	120.4 µs	134.7 µs	-10.57%
❌	Simulation	new_bp_prim_test_between[i32, 16384]	94.8 µs	109.5 µs	-13.4%

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-v2 (3fc3da5) with develop (8c5a0f1)²}

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. ↩

2. No successful run was found on develop (a514cef) during the generation of this report, so 8c5a0f1 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩