simulate items on splitters and fix bugs where buildings could not pull from splitters

src/lib/sim/BeltSystem.h

@@ -108,10 +108,13 @@ private:
     {
         Rotation outputA;
         Rotation outputB;
-        std::vector<ItemType> filterA;  // empty = accept all
+        std::vector<ItemType> filterA;   // empty = accept all
         std::vector<ItemType> filterB;
-        bool nextOutputIsA;             // alternation state
-        std::optional<Item> heldItem;   // item buffered waiting to exit
+        bool nextOutputIsA;              // alternation state
+        std::optional<BeltItemSlot> back;    // progress [0, 0.5]; entering from input belt
+        Rotation                    backDir; // direction of the feeding belt (for animation)
+        std::optional<BeltItemSlot> frontA;  // progress [0, 1]; routed to outputA
+        std::optional<BeltItemSlot> frontB;  // progress [0, 1]; routed to outputB
     };
 
     double m_progressPerTick;  // beltSpeedTilesPerSecond / kTickRateHz