Constant READER_SCALAR_NAMES 

pub(crate) const READER_SCALAR_NAMES: &[&str];

Scalar element types whose Vec<T> (and Vec<Option<T>>) round-trips through TryFromSexp — the supported field types for the parallel from-R reader (try_from_dataframe_par). This is intentionally narrower than FieldTypeKind::Scalar: set/opaque collection types (HashSet<…>, BTreeSet<…>) also classify as Scalar but do NOT implement Vec<_>: TryFromSexp, so they must be excluded from the reader path.

pub(super) so enum_expansion.rs (in the dataframe_derive module dir) can reuse the same allow-list without duplication.