allow to set the recipe already for construction sites

src/lib/sim/Building.h

@@ -54,6 +54,11 @@ struct ConstructionSite
     std::string  recipeId;                      // may be configured before completion
     Tick         completesAt   = 0;             // 0 = queued but not yet started
     std::optional<ShipLayoutConfig> shipLayout;
+
+    // Splitter output filters configured before completion (REQ-BLD-SITE-CONFIG).
+    // Empty = accept all; applied to the BeltSystem when the splitter is built.
+    std::vector<ItemType> splitterFilterA;
+    std::vector<ItemType> splitterFilterB;
 };
 
 // A fully constructed, operational building.