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/MainWindow.h

@@ -66,6 +66,7 @@ private:
     QTableWidget* property_table_ = nullptr;
     QProgressBar* progress_bar_ = nullptr;
     QLabel* status_label_ = nullptr;
+    QLabel* stats_label_ = nullptr;
     QTimer element_poll_timer_;
     QElapsedTimer load_timer_;