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

src/lib/sim/BuildingSystem.h

@@ -43,13 +43,13 @@ public:
                    std::mt19937& rng);
 
     // -- Placement / demolish ------------------------------------------------
-    // Returns the new entity id, or kInvalidBuildingId if the placement falls
-    // outside the world bounds (vertical extent and asteroid left edge).  Belt
-    // and Splitter register with BeltSystem directly; other types enter the
-    // construction queue.  Terrain type (A vs S) is NOT checked here so that
-    // tests can stage arbitrary layouts; the player-facing entry point
+    // Returns the new entity id, or nullopt if the placement falls outside the
+    // world bounds (vertical extent and asteroid left edge).  Belt and Splitter
+    // register with BeltSystem directly; other types enter the construction
+    // queue.  Terrain type (A vs S) is NOT checked here so that tests can stage
+    // arbitrary layouts; the player-facing entry point
     // (Simulation::tryPlaceBuilding) enforces the full rule via isPlacementValid.
-    BuildingId place(BuildingType type, QPoint anchor, Rotation rotation,
+    std::optional<BuildingId> place(BuildingType type, QPoint anchor, Rotation rotation,
                    Tick currentTick);
 
     // Returns true if the placement satisfies REQ-BLD-PLACE-VALID terrain and