implement building system

src/lib/sim/BeltSystem.h

@@ -64,6 +64,10 @@ public:
     //   Returns nullopt if tile is not a belt, direction mismatches, or tile empty.
     std::optional<Item> tryTakeItem(Port port);
 
+    // peekItem: return the type of the leading item without removing it.
+    //   Returns nullopt if tile is not a belt, direction mismatches, or tile empty.
+    std::optional<ItemType> peekItem(Port port) const;
+
     // -- Maintenance ---------------------------------------------------------
     void clearTiles(const std::vector<QPoint>& tiles);   // REQ-UI-BELT-CLEAR
     void tick();