Function derive_simple_factor 

fn derive_simple_factor(
    name: &Ident,
    impl_generics: &ImplGenerics<'_>,
    ty_generics: &TypeGenerics<'_>,
    where_clause: Option<&WhereClause>,
    variants: &Punctuated<Variant, Comma>,
    rename_all: Option<&str>,
) -> Result<TokenStream>

Generate RFactor, MatchArg, IntoR, and TryFromSexp impls for simple (unit variant) enums.

Each variant maps to a 1-based level index and a level name string. Level names are determined by variant ident, optionally transformed by rename_all or overridden by per-variant #[r_factor(rename = "...")].

Uses a OnceLock-cached levels SEXP for efficient repeated conversion.

Returns Err if any variant has fields (only C-style enums are supported in the simple path).