share a single ItemIconCache across the UI

Four separate caches rasterized the same item SVGs, one of them rebuilt on
every recipe-dialog open. MainWindow now owns one cache and hands a
non-owning pointer to HeaderBar, BuildButtonGrid, GameWorldView and
RecipeSelectionDialog.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GH8ZMRY3vhxxXcaUxBqxkk

src/ui/GameWorldView.h

@@ -83,9 +83,12 @@ class GameWorldView : public QOpenGLWidget,
     Q_OBJECT
 
 public:
+    // itemIcons is the window-wide per-item icon cache (REQ-UI-ITEM-ICON); not
+    // owned, must outlive this widget.
     GameWorldView(Simulation* sim, const GameConfig* config,
                   const VisualsConfig* visuals, const std::string& configDir,
-                  const ParsedReplay* replay, QWidget* parent = nullptr);
+                  ItemIconCache* itemIcons, const ParsedReplay* replay,
+                  QWidget* parent = nullptr);
     ~GameWorldView() override;
 
     double getGameSpeed() const;
@@ -309,9 +312,10 @@ private:
     };
     std::map<BuildingType, BuildingIconRenderers> m_buildingIcons;
 
-    // Per-item icon cache (REQ-UI-ITEM-ICON), loaded from <configDir>/../icons/items.
-    // Shared draw path for belt and port items; pixmaps are cached per target size.
-    std::unique_ptr<ItemIconCache> m_itemIcons;
+    // Per-item icon cache (REQ-UI-ITEM-ICON), shared window-wide and owned by
+    // MainWindow. Shared draw path for belt and port items; pixmaps are cached
+    // per target size.
+    ItemIconCache* m_itemIcons;
 
     // Funnels all player input into the single Simulation::apply chokepoint.
     CommandManager m_commandManager;