Function partial_eval_expr 

pub fn partial_eval_expr(
    expr: &Expr,
    sig: &Signature,
    derived_gens: &[Mv],
    arity: usize,
) -> PartialMv

Partially evaluate an Expr, treating specified parameters as polynomial variables and everything else as concrete values.

This is the core of the compilation pipeline: it separates the dynamic probe coordinates from the static algebra structure, producing a PartialMv whose coefficients are polynomials in the probe parameters.