fix issue where items were accepted by a belt from opposite travel direction

src/lib/sim/BeltSystem.h

@@ -74,8 +74,10 @@ public:
     // port.direction = direction items flow on that tile
     //
     // tryPutItem: place item onto tile.
-    //   Returns false if the tile is not a belt/splitter, or tile full.
-    //   fromDir: travel direction of the item (used for splitter animation).
+    //   Returns false if the tile is not a belt/splitter/tunnel entry, tile full,
+    //   or the item would enter through the tile's output edge (REQ-MAT-ACCEPT-DIR).
+    //   fromDir: travel direction of the item (used for splitter animation and for
+    //   the output-edge check).
     bool tryPutItem(QPoint tile, Item item, Rotation fromDir = Rotation::West);
 
     // tryTakeItem: remove and return the leading item from port.tile.
@@ -117,6 +119,12 @@ private:
 
     static std::pair<int, int> key(QPoint tile);
     static QPoint adjacentTile(QPoint tile, Rotation dir);
+    static Rotation oppositeRotation(Rotation dir);
+
+    // True if an item travelling in travelDir would enter the transport tile at
+    // `tile` through one of that tile's output edges (and must therefore be
+    // refused).  Returns false if no transport tile occupies `tile`.
+    bool entersThroughOutputEdge(QPoint tile, Rotation travelDir) const;
 
     // Returns the world-space centre of a slot given tile origin and progress.
     static QPointF slotWorldPos(QPoint tile, Rotation dir, double progress);