move ecs related code to own folder

src/lib/ecs/system/ScrapSystem.h

@@ -0,0 +1,36 @@
+#pragma once
+
+#include <optional>
+#include <vector>
+
+#include <QVector2D>
+
+#include "Tick.h"
+
+#include "entt/entity/entity.hpp"
+
+class EntityAdmin;
+
+struct ScrapInfo
+{
+    entt::entity entity;
+    QVector2D position;
+};
+
+class ScrapSystem
+{
+public:
+    explicit ScrapSystem(EntityAdmin& admin);
+
+    entt::entity spawn(QVector2D position, int amount, Tick despawnAt);
+    void         tickDespawn(Tick currentTick);
+
+    // Removes the scrap and returns its amount, or nullopt if not found.
+    std::optional<int> consume(entt::entity entity);
+
+    // Lightweight snapshot for callers that need to iterate all scrap.
+    std::vector<ScrapInfo> allScrapInfo() const;
+
+private:
+    EntityAdmin& m_admin;
+};