fix(track): 3D sleepers let go of the beams of the tracks far away

world/SleeperWorker.h

@@ -27,5 +27,8 @@ bool in_background();
 void ask_layout( TTrack &Track );
 // nothing left to work out in the background nor being worked out, the waiting ones dropped
 void settle_background();
+// the layout of the cluster kept for the track let go, once the track laid its beams or went far; the cluster stays while
+// another track of it keeps it
+void forget_layout( TTrack const &Track );
 
 } // namespace track_sleepers