store belts as buildings and fix issue that belts could not be selected

src/lib/sim/BuildingSystem.h

@@ -23,8 +23,8 @@
 
 // Manages building placement, construction queuing, and the per-tick
 // production loop (belt→building pull, production, building→belt push).
-// Belt and Splitter types are forwarded to BeltSystem rather than stored
-// as Building instances.
+// All types including Belt and Splitter are stored as Building instances;
+// BeltSystem owns the per-tile simulation data (item slots, flow).
 class BuildingSystem
 {
 public:
@@ -106,14 +106,6 @@ public:
     void forEachBuilding(std::function<void(Building&)> fn);
 
 private:
-    struct BeltEntry
-    {
-        QPoint       tile;
-        BuildingType type;       // Belt or Splitter
-        Rotation     directionA; // Belt: its direction; Splitter: first output
-        Rotation     directionB; // Splitter: second output; Belt: same as directionA
-    };
-
     const BuildingDef*  findBuildingDef(BuildingType type) const;
     const RecipeDef*    findRecipe(const std::string& id, BuildingType type) const;
     const ShipDef*      findShipDef(const std::string& id) const;
@@ -131,7 +123,6 @@ private:
 
     std::vector<Building>           m_buildings;
     std::deque<ConstructionSite>    m_constructionQueue;
-    std::map<EntityId, BeltEntry>   m_beltEntities;
 
     // Maps every occupied body-cell coordinate to the entity that owns it.
     std::map<std::pair<int, int>, EntityId> m_tileOccupancy;