Function unit_factor_option_vec_from_sexp 

pub fn unit_factor_option_vec_from_sexp<T: UnitEnumFactor>(
    sexp: SEXP,
) -> Result<Vec<Option<T>>, SexpError>

Convert an R factor SEXP to a Vec<Option<T>> using UnitEnumFactor (NA → None).

Used by the enum DataFrame reader to reconstruct as_factor columns. Unlike factor_option_vec_from_sexp (which requires RFactor + MatchArg), this accepts any UnitEnumFactor — including #[derive(DataFrameRow)] unit-only enums that do not implement RFactor.