Function compose_unit_symbols 

pub fn compose_unit_symbols(
    name: &str,
    local_table: &UnitTable,
    unit_info: &BTreeMap<String, UnitInfo>,
) -> (HashMap<String, Arc<TypeDecl>>, HashMap<String, Arc<FnDecl>>, HashMap<String, MethodTable>, HashMap<String, String>, HashMap<String, UnitKind>)

Phase 8a: compose one unit’s symbol space — its local table plus a one-level uses mixin (commons identity preserved). Returns the combined type/fn/method tables and the imported_from provenance maps; the mixin loop has no continues, so it lifts out whole.