Function effective_r_defaults 

fn effective_r_defaults(
    method: &ParsedMethod,
    c_ident: &str,
) -> HashMap<String, String>

Effective R-formal defaults for a method.

Layers defaults in priority order:

1. #[miniextendr(match_arg)] → ALWAYS a write-time placeholder that the cdylib resolves to c("a", "b", ...) at package-load time. Any user- supplied default = "X" is consumed elsewhere (rotates X to the front of the choice list at write time) rather than overriding the formal.
2. #[miniextendr(choices("a", "b", ...))] → c("a", "b", ...) formal default.
3. User-provided #[miniextendr(defaults(param = "..."))] for non-match_arg params.