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

src/lib/sim/Simulation.h

@@ -2,6 +2,7 @@
 
 #include <map>
 #include <memory>
+#include <optional>
 #include <random>
 #include <set>
 #include <string>
@@ -131,8 +132,8 @@ private:
     // Reached during play exclusively via apply(); never called by UI/app code.
 
     // Checks affordability, deducts building blocks, and places the building.
-    // Returns the new entity id, or kInvalidBuildingId if blocks are insufficient.
-    BuildingId tryPlaceBuilding(BuildingType type, QPoint anchor, Rotation rotation);
+    // Returns the new entity id, or nullopt if blocks are insufficient.
+    std::optional<BuildingId> tryPlaceBuilding(BuildingType type, QPoint anchor, Rotation rotation);
 
     // Demolishes the building with the given id and refunds building blocks.
     void demolish(BuildingId id);
@@ -182,7 +183,7 @@ private:
     int      m_artifactCount = 0;
 
     // Pre-placed structure IDs.
-    BuildingId     m_hqBuildingId;           // Building id (for belt integration)
+    std::optional<BuildingId> m_hqBuildingId;  // Building id (for belt integration)
     entt::entity m_hqProxyEntity;          // ECS entity (HP, targeting)
     entt::entity m_playerStation1Entity;
     entt::entity m_playerStation2Entity;