Function is_reader_scalar_ty 

pub(crate) fn is_reader_scalar_ty(ty: &Type) -> bool

True if ty is a known scalar path, or Option<known-scalar>.

These are exactly the field types for which the from-R reader can pull a column out as Vec<ty> via TryFromSexp (scalar Single fields and [T; N] fixed-array elements, neither of which adds an Option wrapper).

pub(super) so enum_expansion.rs can reuse it.