disk-load retires the readings of a module whose key went while its watch was down (review round 2)

A key deleted while disk-load was not watching (a restart, a reboot, a machine asleep) never
arrives as a delete. Once the watch has handed over every current key, the readings of modules
with no key are retired once; an empty set is skipped and said, since the engine has not written.

Claude-Session: https://claude.ai/code/session_01NcbgfJ9Gum47zkFKWRaovL

modules/disk-load/cmd/disk-load/data.go

@@ -115,6 +115,8 @@ func (w *Watched) Follow(k *Keeper) error {
 		return nil
 	})
 	w.Ready()
+	// The handed-over set is complete now: a module whose key went while the watch was down is retired once.
+	k.RetireUnkeyed(context.Background())
 	// A round refused while the watch caught up runs now, not at the next hour (the delivery's readings within minutes).
 	go k.RoundIfRefused(context.Background())
 	return nil