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

src/lib/ecs/component/DeliverScrapBehavior.h

@@ -1,5 +1,7 @@
 #pragma once
 
+#include <optional>
+
 #include "BuildingId.h"
 
 // Deliver-scrap behavior (one half of the old SalvageBehaviorComponent). Scored
@@ -7,6 +9,6 @@
 // SalvagerSystem performs the actual delivery.
 struct DeliverScrapBehavior
 {
-    BuildingId deliveryBay = kInvalidBuildingId;
-    float      score = 0.0f;
+    std::optional<BuildingId> deliveryBay;  // nullopt until a bay is assigned
+    float                     score = 0.0f;
 };