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