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.

internal/inventory/forget_test.go

@@ -212,7 +212,7 @@ func TestAModuleStillAssignedRefusesBeforeAnythingAboutWhatItHolds(t *testing.T)
 	if err := inv.SetSettings(ctx, "anchor", "step-ca", map[string]any{"a": 1}); err != nil {
 		t.Fatal(err)
 	}
-	if err := inv.Assign(ctx, "anchor", "step-ca"); err != nil {
+	if _, err := inv.Assign(ctx, "anchor", "step-ca"); err != nil {
 		t.Fatal(err)
 	}
 	for _, forget := range []func() error{