implement tunnels

src/lib/config/BuildingsConfig.h

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <optional>
 #include <string>
 #include <vector>
 
@@ -18,6 +19,7 @@ struct BuildingDef
     // Stored as raw strings here; parsing into per-cell tiles + output ports
     // happens when buildings are placed, not at load time.
     std::vector<std::string> surfaceMask;
+    std::optional<int> tunnelMaxDistance;
 };
 
 struct BuildingsConfig