perf(track): the gauge lines, crossings and check rails of a track are kept until a track is made, rebuilt or removed, so laying the rails of a track no longer recomputes its neighbours' crossings for each wing rail; self-test command railtime times planning the rails of all tracks

world/TrackRails.h

@@ -20,6 +20,7 @@ double constexpr nose_tip { 0.012 };
 double constexpr middle_angle { 0.35 };
 double constexpr wide_angle { 0.785 };
 
+void forget();
 // how far a wing rail reaches past the crossing point of the rails
 double wing_reach( double const Slope );
 double check_length( double const Slope );