Function resolve_struct_field 

fn resolve_struct_field(
    field: &Field,
    index: usize,
    is_tuple: bool,
) -> Result<Option<ResolvedField>>

Resolve a struct field into a ResolvedField, applying field attributes.

Combines the field’s #[dataframe(...)] attributes with its type classification to determine the codegen strategy:

• skip -> returns None
• as_list -> Single (suppresses expansion)
• FixedArray -> ExpandedFixed (compile-time expansion to N columns)
• VariableVec/BoxedSlice/BorrowedSlice + width -> ExpandedVec
• VariableVec/BoxedSlice/BorrowedSlice + expand -> AutoExpandVec
• Everything else -> Single

Returns Err if width or expand is used on an incompatible type.