knowledge-base: keep a retraction from being undone by a later acceptance or a redraft

A record proposed before a retraction and accepted after it marked the
retracted clause changed or the retracted record superseded, serving it again;
and returning a drafted record numbered its clauses anew, bringing retracted
ones back in force or making a waiting Approve mark another clause. accept now
leaves what is retracted alone, Return refuses a record with a retraction, and
retract waits until a record's drafting is done.

modules/mesh-knowledge/cmd/mesh-knowledge/draft.go

@@ -500,6 +500,16 @@ func (k *KB) Return(caller string, key Key, reason string) (map[string]any, erro
 		return nil, fmt.Errorf("the knowledge base holds no %s", key)
 	}
 	r := e.Current.Clone()
+	if r.Retracting != nil || len(r.Retractions) > 0 {
+		// Drafting again numbers the clauses anew and brings them back in force: a retraction waiting would mark
+		// another clause than the operator was shown, and one approved would be undone.
+		which := "approved"
+		if r.Retracting != nil {
+			which = "waiting"
+		}
+		return nil, fmt.Errorf("%s is not returned: it has a retraction %s, which drafting it again would undo or point at other "+
+			"clauses. Withdraw a waiting retraction first; a retracted record or clause stays so", key, which)
+	}
 	if r.Drafting == nil {
 		return nil, fmt.Errorf("%s's clauses were written by its author, not drafted: a written clause is changed by a new record", key)
 	}