Lambda and Variable Expressions and BoundExpressions

[mhk197]

Adds lambdas and variables as Expression Variants and implements the scope machinery to bind them.

/// A name bound in a scope.
pub struct Variable(Arc<str>);

/// A body evaluated under a frame binding `params`.
pub struct Lambda { params: Box<[Variable]>, body: Arc<Expression> }

pub enum Expression {
    Scalar { scalar_fn: ScalarFnRef, children: Arc<Vec<Expression>> },
    Root,
    Variable(Variable),
    Lambda(Lambda),
}

pub struct Scope { root: DType, frames: Vec<Frame> }   // Frame: [(Variable, DType)]

Binding:

expr.bind_scope(&scope)                      // errors on a lambda in a value position
lambda.bind(&scope, param_dtypes)            // -> BoundLambda
scope.push_frame(frame)                      // a caller can bind names without any binder node

Design notes

A lambda is bound by whoever knows its parameter types. They come from whatever applies it, so
Lambda::bind takes them. That is the entry point a higher-order function will call with types
derived from its own arguments; here, tests call it directly.

Scope carries frames, not a flat map. A flat name -> dtype map cannot distinguish "this
binder's own parameter" from "a name from an enclosing binder", which is what a capture check needs.
resolve returns the dtype and the frame depth so that check is later a comparison rather than a
re-derivation.

[codspeed-hq]

Merging this PR will regress 1 benchmark

⚡ 3 improved benchmarks
❌ 1 regressed benchmark
✅ 1925 untouched benchmarks
⏩ 51 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	chunked_dict_primitive_canonical_into[f32, (1000, 100, 100)]	1.5 ms	1.6 ms	-11.15%
⚡	Simulation	take_map[(0.1, 0.5)]	2.5 ms	2.2 ms	+14.29%
⚡	Simulation	take_map[(0.1, 1.0)]	4 ms	3.6 ms	+10.91%
⚡	Simulation	chunked_varbin_opt_to_varbinview_builder[(10, 100)]	842.9 µs	764.8 µs	+10.21%

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 mk/lambda-variables (8e710f5) with mk/bound-expression-enum (9296dac)}

Footnotes

1. 51 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. ↩