Answer architecture round 1: one owner keeps a folder's loaded and unloaded names apart, each loaded category is checked against its index entry, addSource returns what it replaced, the manifest cache goes, ErrLoad exits 1, and the docs say what New and Fake now refuse

inline.go

@@ -91,7 +91,7 @@ func loadInline(root *folder, n node, label string, typedByGo bool) error {
 		return err
 	}
 	if err := loadReached(root, nil, referenced(root, nil, nodes)); err != nil {
-		return err
+		return fmt.Errorf("%w: %w", ErrLoad, err)
 	}
 	return pipeline{
 		nodes:     nodes,