phraser: name the service that is down (V-444)

Stub and LLM paths both read loop.DownServices, so the message can never name
a service the predicate did not fire on. Two down at once are both named — he
needs the blast radius.

cmd/mavend/tick_test.go

@@ -584,7 +584,7 @@ func TestDigestSev4BypassesQueue(t *testing.T) {
 	ctx := context.Background()
 	now := refNow()
 	markPresent(t, st, ctx, now)
-	if _, err := st.SetValue(ctx, store.KindSelf, "service_down", "poll:uptimekuma", "down", now); err != nil {
+	if _, err := st.SetValue(ctx, store.KindSelf, "service_down:db", "poll:uptimekuma", "down", now); err != nil {
 		t.Fatalf("seed service_down: %v", err)
 	}
 	sink := &fakeSink{}