Function is_altrep_struct 

pub fn is_altrep_struct(item: &ItemStruct) -> bool

Check if a struct with #[miniextendr] should be treated as ALTREP (needing struct Name; in module).

Returns true only for 1-field structs without explicit mode attrs (list, dataframe, externalptr). Multi-field structs, structs with explicit mode attrs, and enums don’t need module entries.