allow to rotate buildings in place

src/lib/sim/BuildingSystem.h

@@ -74,6 +74,18 @@ public:
     std::vector<BeltTileInfo> allBeltTiles()             const;
     bool                      isTileOccupied(QPoint tile) const;
 
+    // Returns the entity id of the building or construction site whose footprint
+    // exactly coincides with the ghost (type, anchor, rot) and is of the same
+    // building type.  Returns nullopt otherwise.
+    std::optional<EntityId> findRotateInPlaceTarget(BuildingType type,
+                                                     QPoint anchor,
+                                                     Rotation rot) const;
+
+    // Rotate an existing building or construction site to newRotation in place.
+    // For belt-type operational buildings, re-registers with BeltSystem (items
+    // currently on the tile are discarded by BeltSystem::removeTile).
+    void rotateInPlace(EntityId id, Rotation newRotation);
+
     // Find nearest operational building of the given type; nullptr if none.
     const Building* findNearestBuilding(QVector2D worldPos, BuildingType type) const;