Refactor Expression Optimizer

[mhk197]

Summary

• add a deterministic bottom-up optimizer for bound expressions
• preserve expression dtypes across rewrites and reject invalid or unchanged replacements
• organize built-in rewrites by binary, cast, structural, null, and conditional families

Changes

• add a reusable BoundExpressionOptimizer with ordered rule registration and a rewrite-cycle budget
• use a per-invocation OptimizationRun with small inline task and result stacks
• rebuild parent nodes only when an optimized child changed
• mirror the existing scalar-function simplify and reduce behaviors with bound-expression rules
• document the optimizer state machine and every built-in rewrite with original and rewritten examples

Testing

• cargo test -p vortex-array expr::optimizer
• cargo test --doc -p vortex-array
• cargo clippy --all-targets --all-features
• cargo +nightly fmt --all -- --check
• git diff --check

[codspeed-hq]

Merging this PR will improve performance by 27.15%

⚠️ 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.

⚡ 3 improved benchmarks
✅ 490 untouched benchmarks
⏩ 1725 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	WallTime	arrow_checked_add_u32_neon[16384]	20.4 µs	13.6 µs	+49.86%
⚡	WallTime	arrow_checked_add_u32_avx512[16384]	21.2 µs	17.7 µs	+20.35%
⚡	WallTime	words_gather_scalar_avx2[65536]	9.5 µs	8.3 µs	+13.98%

Tip

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

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_{Comparing mk/bound-expression-optimizer (314fb23) with develop (34a6912)²}

Footnotes

1. 1725 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 (b240a50) during the generation of this report, so 34a6912 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩