Add performance stats overlay in status bar

Show FPS, frame time, visible/total objects, and visible/total
triangles in the status bar. Toggled via Settings > Show Performance
Stats, persisted in app settings.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/ifcviewer/SettingsWindow.h

@@ -22,6 +22,7 @@
 
 #include <QDialog>
 
+class QCheckBox;
 class QLineEdit;
 class QShowEvent;
 
@@ -41,6 +42,7 @@ private:
     void syncFromSettings();
 
     QLineEdit* geometry_library_edit_ = nullptr;
+    QCheckBox* show_stats_check_ = nullptr;
 };
 
 #endif