[18RoyalGorge] Require graph connection for cross-buying trains

From page 13 of 24 in the [current rulebook PDF][0].

Check for the connection when processing the action--instead of when finding
available trains to buy--to minimize the necessary graph checks.

Fixes tobymao#11364

[0]: https://boardgamegeek.com/filepage/284918/graphic-rulebook-preliminary

lib/engine/game/g_18_royal_gorge/step/buy_train.rb

@@ -16,6 +16,21 @@ def room?(entity)
             # even when train tight, there's room for a self-rust
             return true if entity.trains.any? { |t| t.rusts_on == upcoming_train.name }
           end
+
+          def check_connected!(buyer, train)
+            return if train.owner == @game.depot
+
+            seller = train.corporation
+            seller_nodes = @game.graph.connected_nodes(seller)
+            return if @game.graph.connected_nodes(buyer).any? { |node, _| seller_nodes.include?(node) }
+
+            raise GameError, "#{buyer.name} must be connected to #{seller.name} to buy their train."
+          end
+
+          def process_buy_train(action)
+            check_connected!(action.entity, action.train)
+            super
+          end
         end
       end
     end