Function build_r_call_args_from_sig 

pub(crate) fn build_r_call_args_from_sig(sig: &Signature) -> String

Build R .Call() arguments from a Rust function signature.

Automatically skips self/&self receivers (those are passed separately via DotCallBuilder::with_self). Dots become list(...).

Returns a comma-separated string of R call arguments, e.g., "x, y, list(...)".