Use std::optional instead of sentinel values for absent data

src/lib/sim/BuildingSystem.cpp

@@ -346,7 +346,7 @@ std::vector<Item> BuildingSystem::rollReprocessingOutput(const RecipeDef& recipe
 // Placement
 // ---------------------------------------------------------------------------
 
-BuildingId BuildingSystem::place(BuildingType type, QPoint anchor,
+std::optional<BuildingId> BuildingSystem::place(BuildingType type, QPoint anchor,
                                 Rotation rotation, Tick currentTick)
 {
     const BuildingDef* def = findBuildingDef(type);
@@ -356,7 +356,7 @@ BuildingId BuildingSystem::place(BuildingType type, QPoint anchor,
     // Reject placements that fall outside the world (REQ-BLD-PLACE-VALID).
     if (!bodyCellsWithinWorldBounds(mask.bodyCells, anchor))
     {
-        return kInvalidBuildingId;
+        return std::nullopt;
     }
 
     const BuildingId id = m_allocateBuildingId();