allow to (multi) select scrap

src/ui/GameWorldView.h

@@ -168,6 +168,13 @@ private:
     void     placeBlueprintAtTile(QPoint center);
 
     std::optional<QVector2D> entityPosition(entt::entity entity) const;
+    // Clears the scrap selection, emitting an empty ScrapSelectionChangedEvent when
+    // it was non-empty (REQ-UI-SCRAP-CLICK-SELECT). Used when another selection
+    // category takes over.
+    void                     clearScrapSelection();
+    // Drops despawned or fully-collected piles from the scrap selection and re-emits
+    // when it changed (REQ-UI-SCRAP-CLICK-SELECT). Called each frame from onFrame().
+    void                     pruneDespawnedScrap();
     void                     stepSpeed(int delta);
     void                     placeAtTile(QPoint tile);
 
@@ -249,6 +256,7 @@ private:
 
     std::vector<BuildingId> m_selectedBuildingIds;
     std::optional<entt::entity> m_selectedEntity;
+    std::vector<entt::entity> m_selectedScrap;
     bool                  m_boxSelecting;
     QPoint                m_boxStartTile;
     QPoint                m_boxCurrentTile;