Function prepare_unit_check_ctx 

pub fn prepare_unit_check_ctx(
    kind: UnitKind,
    unit_info: &BTreeMap<String, UnitInfo>,
    combined_types: &HashMap<String, Arc<TypeDecl>>,
    imported_from_kind: &HashMap<String, UnitKind>,
) -> UnitCheckCtx

Build the per-unit prelude check_file_core shares across every file in the unit — see UnitCheckCtx’s own doc comment.