A repeat assignment says nothing changed (ADR 0115)

One assignment of a module per node is now the rule, not a limitation —
the operator dropped the multi-assignment requirement, and the schema's
(node, module) key has been the decision since migration 0005. What
changed: Assign reports whether the assignment was new, and the command
says 'already runs — one node runs one of each (ADR 0115); nothing
changed' instead of printing 'is assigned' for a no-op, which read as
an action that happened. Idempotence stays: a repeat is exit 0, because
a script stating what is already true is not wrong.

cmd/mesh-controller/mesh_for_test.go

@@ -73,7 +73,7 @@ func aMesh(t *testing.T) *stores {
 		if err := open.inventory.RecordOverlayKey(t.Context(), record.ID, aPublicKey(t)); err != nil {
 			t.Fatal(err)
 		}
-		if err := open.inventory.Assign(t.Context(), name, overlay.Name); err != nil {
+		if _, err := open.inventory.Assign(t.Context(), name, overlay.Name); err != nil {
 			t.Fatal(err)
 		}
 	}