Function build_r_formals_from_sig 

pub(crate) fn build_r_formals_from_sig(
    sig: &Signature,
    defaults: &HashMap<String, String>,
) -> String

Build R formal parameters from a Rust function signature, with optional defaults.

Automatically skips self/&self receivers. Missing<T> parameters without user-provided defaults appear as bare formals (no default value); the if (missing()) prelude is generated separately via build_missing_prelude.

Returns a comma-separated string of R formals, e.g., "x, y = NULL, ...".