Function expand_enum 

fn expand_enum(
    attr: TokenStream,
    item: TokenStream,
    item_enum: &ItemEnum,
) -> TokenStream

Dispatches #[miniextendr] on a fieldless enum to the correct derive path.

Only C-style (fieldless) enums are supported. Dispatches to:

• match_arg mode: MatchArg derive (single-selection from R character scalar)
• factor mode or default: RFactor derive (R factor representation)