implement ship behaviors

src/lib/sim/ScrapSystem.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include <functional>
+#include <optional>
 #include <vector>
 
 #include <QVector2D>
@@ -14,8 +15,9 @@ class ScrapSystem
 public:
     explicit ScrapSystem(std::function<EntityId()> allocateId);
 
-    EntityId spawn(QVector2D position, int amount, Tick despawnAt);
-    void     tickDespawn(Tick currentTick);
+    EntityId             spawn(QVector2D position, int amount, Tick despawnAt);
+    void                 tickDespawn(Tick currentTick);
+    std::optional<Scrap> consume(EntityId id);  // removes and returns scrap, or nullopt
 
     const Scrap*       findScrap(EntityId id) const;
     std::vector<Scrap> allScraps()            const;