Close a batch's merge window by the expiry of a key on the bus, heard by a watcher, and drop the five-second cutter (issue 973)

cmd/mesh-controller/push.go

@@ -183,9 +183,12 @@ func serve(ctx context.Context) (err error) {
 	// Open plans move on a timer as well as on outcomes (novox/hq ADR 0162): a tier waiting for
 	// machines to report moves when they have, and a plan left by a replaced controller resumes.
 	go planTicker(ctx, open)
-	// And the merge window (novox/hq ADR 0276): a batch is cut into its walk when the window closes, which no
-	// merge or outcome says.
-	go batchCutter(ctx, open)
+	// And the merge window (novox/hq ADR 0276, ADR 0407): a batch is cut into its walk when the window closes,
+	// which no merge or outcome says. The window is two keys on the bus that expire and a watcher that hears it;
+	// nothing polls (issue 973).
+	if err := startMergeWindow(ctx, open, server.JetStream()); err != nil {
+		return fmt.Errorf("the merge window could not be put on the bus: %w", err)
+	}
 	// And the pending assignments settled on a tick of their own (novox/hq ADR 0261): made once their module
 	// is registered, ended with why when its build will not register it, raised and cleared as conditions.
 	// Never by a read.