The controller takes one announcement at a time

A merge's handler builds for minutes and keeps its own delivery alive; the announcements handed over
behind it timed out on the client and came back, and a merge that came back rebuilt what it had just
built, five times over (novox/hq issue 175). MaxAckPending 1 on the events consumer: the server holds
the rest.

internal/broker/derived.go

@@ -40,7 +40,14 @@ type Consumer struct {
 	AckWaitSeconds int
 	// MaxDeliver before the message is dead-lettered; zero for the mesh's default.
 	MaxDeliver int
-	Why        string
+	// MaxAckPending is how many deliveries the server lets stand unacknowledged at once; zero for
+	// the server's default, which is many. **One, for a consumer handled one at a time**
+	// (novox/hq issue 175): a handler that builds for minutes keeps its own message alive with a
+	// heartbeat, but everything handed over behind it times out unacknowledged and comes back —
+	// and a merge that came back rebuilt what it had just built, five times over on 2026-09-30.
+	// With one outstanding, the server holds the rest, and the heartbeat is keeping the message.
+	MaxAckPending int
+	Why           string
 }
 
 // seatStreamName is the stream holding a seat's inbound work. Named after the seat rather than

internal/broker/jetstream.go

@@ -179,6 +179,7 @@ func (j *JetStream) EnsureConsumer(c Consumer) error {
 		AckPolicy:      nats.AckExplicitPolicy,
 		AckWait:        time.Duration(c.AckWaitSeconds) * time.Second,
 		MaxDeliver:     c.MaxDeliver,
+		MaxAckPending:  c.MaxAckPending,
 		DeliverGroup:   c.Queue,
 		DeliverSubject: "",
 		Description:    c.Why,

internal/broker/streams.go

@@ -244,8 +244,13 @@ func MeshConsumers() []Consumer {
 			Push:           true,
 			AckWaitSeconds: 30,
 			MaxDeliver:     5,
-			Why: "the two events the mesh's own controller reacts to; after max-deliver it " +
-				"dead-letters, because an announcement it cannot act on will not become actionable",
+			// One at a time (novox/hq issue 175): acting on a merge builds for minutes, and an
+			// announcement handed over behind it must wait on the server, not time out on the
+			// client and come back to be acted on again.
+			MaxAckPending: 1,
+			Why: "the two events the mesh's own controller reacts to, one at a time; after " +
+				"max-deliver it dead-letters, because an announcement it cannot act on will not " +
+				"become actionable",
 		},
 	}
 }

internal/broker/streams_test.go

@@ -260,3 +260,14 @@ func TestNoTwoConsumersDeliverOntoTheSameSubject(t *testing.T) {
 		seen[subject] = c.Name + " on " + c.Stream
 	}
 }
+
+// The controller's events consumer is handed one announcement at a time (novox/hq issue 175): a
+// merge's handler builds for minutes, and what is queued behind it must wait on the server rather
+// than time out on the client and be acted on twice.
+func TestTheControllerTakesOneAnnouncementAtATime(t *testing.T) {
+	for _, c := range MeshConsumers() {
+		if c.Stream == "EVENTS" && c.Name == ControllerName && c.MaxAckPending != 1 {
+			t.Fatalf("the events consumer may have %d outstanding; one announcement at a time", c.MaxAckPending)
+		}
+	}
+}