Derive ship scrap drop from threat

src/lib/ecs/component/ShipIdentityComponent.h

@@ -6,4 +6,7 @@ struct ShipIdentityComponent
 {
     int level;
     std::string schematicId;
+    // Scrap dropped on destruction, derived from the ship's as-built threat cost
+    // at spawn time (REQ-RES-SCRAP-DROP).
+    int scrapDrop = 0;
 };