Lambda/var expression to impl HoF

[joseph-isaacs]

We want to introduce lambda and variable expressions to support higher-order functions in the Vortex expression tree.

Static Lambda Expressions

Vortex expressions today form a tree of scalar function nodes, where Root references the input array and every other node computes a value from its children. There is no way to name an intermediate result or express a parameterized subcomputation within the expression tree. Lambda and Variable expressions introduce these capabilities: lambda("x", checked_mul(var("x"), var("x"))) defines a function over x, and var("x") references the bound parameter.

A Lambda is a scalar function with a FieldName option (the parameter name) and exactly one child (the body). A Variable is a zero-arity leaf node with options { name: FieldName, dtype: DType } that references a named parameter. Both are non-serializable and non-executable — they are purely structural constructs that must be eliminated before reaching the execution layer.

Elimination happens statically in ArrayRef::apply. Before the existing Root/Literal/general logic runs, a top-down tree rewrite removes all lambda and variable nodes by substituting variable references with the appropriate bound expressions. This produces a plain lambda-free expression tree that the rest of apply handles normally. Lambda::simplify_untyped performs the same substitution for expression-level optimization passes that operate before arrays exist.

Execution
Lambda and Variable expressions cannot be executed directly today — execute will bail if either survives to the execution layer. In the future, we could support execution by threading a scope map (HashMap<FieldName, ArrayRef>) through apply, allowing a lambda to evaluate its bound value once and resolve variable references to the precomputed array.