a turn hands back its trace id, and one wire op corrects it (V-630)

The correction is the only supervised signal in the box, so the cost of
giving one has to be near zero. That means the surface needs the trace id of
the turn it is showing, which it had no way to learn: handleText returns one
string and the trace was written after the reply left.

The id rides back on ChatReply through the same context sink querySource
uses, so the mic, telegram and the web keep the one signature they share.
CorrectTurn takes a trace id and an optional target, which is deliberately
reach-agnostic: nothing about it assumes a browser.

store.ErrNoSuchTrace gets a wire twin. A turn past the retention bound is
gone, and that is the expected outcome of correcting an old turn, not a
broken database.

cmd/mavend/routingtrace_test.go

@@ -77,3 +77,49 @@ func TestEmptyUtteranceIsNotPersisted(t *testing.T) {
 }
 
 var _ traceWriter = (*store.Store)(nil)
+
+// The trace id rides back to the caller, which is what makes a correction one
+// gesture: /chat already has the id, so saying "that was wrong" costs a button
+// and no lookup (V-630).
+func TestTurnHandsBackItsTraceID(t *testing.T) {
+	h := traceHandler(t, decision.NewRing())
+	h.traces = traceSink(h.dataStore)
+
+	ctx, sink := withTraceIDSink(context.Background())
+	if reply := h.handleText(ctx, "web", "сколько сейчас времени"); reply == "" {
+		t.Fatal("turn produced no reply")
+	}
+	id := sink.ID()
+	if id == 0 {
+		t.Fatal("no trace id came back, so /chat can offer no correction")
+	}
+	// And it names the turn that just ran, so the correction lands on the right
+	// utterance.
+	if err := h.dataStore.CorrectTurn(context.Background(), id, "query", h.now()); err != nil {
+		t.Fatal(err)
+	}
+	labels, err := h.dataStore.RoutingLabels(context.Background(), 5)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(labels) != 1 || labels[0].Utterance != "сколько сейчас времени" {
+		t.Fatalf("labels %+v, want the turn that just ran", labels)
+	}
+}
+
+// A turn nobody asked the id of costs nothing, which is the mic path.
+func TestTurnWithNoSinkStillPersists(t *testing.T) {
+	h := traceHandler(t, decision.NewRing())
+	h.traces = traceSink(h.dataStore)
+
+	if reply := h.handleText(context.Background(), "web", "сколько сейчас времени"); reply == "" {
+		t.Fatal("turn produced no reply")
+	}
+	got, err := h.dataStore.RecentRoutingTraces(context.Background(), 5)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(got) != 1 {
+		t.Fatalf("persisted %d traces, want 1", len(got))
+	}
+}