feat(expr): add bound expressions

[joseph-isaacs]

Summary

• Introduce Scope and a type-checked BoundExpression tree whose nodes retain their inferred dtypes.
• Add fallible construction, child rebuilding, binding and unbinding, structural and exact-identity equality, iterative drop, and tree display support.
• Keep Expression::return_dtype as a compatibility path backed by expression binding.

Why

Consumers that know the authoritative root dtype should be able to bind once and retain type information instead of recursively recomputing it throughout analysis and execution.

Impact

This is PR 1 of 2 in the stack. It adds the bound-expression representation and compatibility API without changing scan or layout interfaces.

Stack

1. This PR: bound-expression foundation
2. refactor: propagate bound expressions through layouts #9126: array execution and scan/layout propagation

Validation

• cargo check -p vortex-array
• cargo +nightly fmt --all -- --check
• git diff --check

[codecov]

Codecov Report

❌ Patch coverage is 84.21053% with 42 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.36%. Comparing base (a6b4527) to head (bcb3aea).
⚠️ Report is 6 commits behind head on develop.

Files with missing lines	Patch %	Lines
vortex-array/src/expr/bound_expression.rs	80.47%	41 Missing ⚠️
vortex-array/src/expr/display.rs	97.43%	1 Missing ⚠️

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[codspeed-hq]

Merging this PR will degrade performance by 48.47%

❌ 4 regressed benchmarks
✅ 1838 untouched benchmarks
⏩ 55 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	pack_return_dtype[2000]	1.3 ms	2.5 ms	-50.68%
❌	Simulation	pack_return_dtype[500]	318 µs	614.1 µs	-48.22%
❌	Simulation	pack_return_dtype[1000]	633.8 µs	1,223.9 µs	-48.21%
❌	Simulation	pack_return_dtype[100]	67.3 µs	126.3 µs	-46.69%

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 ji/bound-expr-foundation (bcb3aea) with develop (a1c2853)}

Footnotes

1. 55 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. ↩