Snap belt-drag end tile to a building's input edge

src/lib/sim/BuildingSystem.h

@@ -171,6 +171,12 @@ public:
     // Find nearest operational building of the given type; nullptr if none.
     const Building* findNearestBuilding(QVector2D worldPos, BuildingType type) const;
 
+    // Input-capable adjacent tiles for a building or construction site
+    // (REQ-BLD-BELT-DRAG, REQ-MAT-INPUT-PORTS): each returned Port.tile is the
+    // outside adjacent tile and Port.direction is the belt facing that points into
+    // the target. Output-port edges are excluded. Empty for an unknown id.
+    std::vector<Port> getInputPorts(BuildingId id) const;
+
     // Register / unregister tile occupancy for ECS station entities.
     void registerTileOccupancy(const std::vector<QPoint>& cells, BuildingId ownerPlaceholder);
     void unregisterTileOccupancy(const std::vector<QPoint>& cells);
@@ -246,6 +252,9 @@ private:
     void                initShipyardBuffers(Building& b) const;
     void                initSalvageBayBuffer(Building& b) const;
     std::vector<Port>   computeInputPorts(const Building& b) const;
+    // Core input-edge scan shared by operational buildings and construction sites.
+    std::vector<Port>   computeInputPorts(const std::vector<QPoint>& bodyCells,
+                                          const std::vector<Port>& outputPorts) const;
     std::vector<Item>   rollReprocessingOutput(const RecipeDef& recipe);
     bool                bodyCellsWithinWorldBounds(
                             const std::vector<QPoint>& bodyCells,