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

src/lib/config/ConfigLoader.cpp

@@ -275,6 +275,11 @@ BuildingsConfig ConfigLoader::loadBuildings(const std::string& path)
         def.constructionTimeSeconds = requireDouble(mt["construction_time_seconds"], file, elemPath + ".construction_time_seconds");
         def.surfaceMask             = requireStringArray(mt["surface_mask"],         file, elemPath + ".surface_mask");
 
+        if (const std::optional<int64_t> tmd = mt["tunnel_max_distance"].value<int64_t>())
+        {
+            def.tunnelMaxDistance = static_cast<int>(*tmd);
+        }
+
         const std::optional<BuildingType> parsedType = parseBuildingType(def.id);
         if (!parsedType)
         {