ifcviewer: event-driven rendering, idle scenes cost zero CPU

Replaced the 16ms QTimer with QEvent::UpdateRequest delivered via
requestUpdate(), posted from every state mutator (mouse/wheel, model
lifecycle, selection, visibility, resize).  A static BIM scene — the
common case for a viewer — now does no work at all between user actions.

FPS is now measured as time spent inside render() rather than wall-clock
gap between frames, so idle gaps don't pollute the 1-second window and
the headline number reflects real render throughput.  Headline fps still
caps at vsync; sub-vsync profiling lives in the cull[...] phase timers.

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

src/ifcviewer/ViewportWindow.h

@@ -23,7 +23,6 @@
 #include <QWindow>
 #include <QOpenGLContext>
 #include <QtOpenGL/QOpenGLFunctions_4_5_Core>
-#include <QTimer>
 #include <QElapsedTimer>
 #include <QMatrix4x4>
 #include <QVector3D>
@@ -107,6 +106,11 @@ struct ModelGpuData {
     bool hidden    = false;
 };
 
+// Rendering is event-driven: render() runs only when QEvent::UpdateRequest
+// is delivered, posted via requestUpdate().  An idle scene costs zero CPU.
+// INVARIANT: every public mutator that changes what should be on screen
+// (camera, selection, model lifecycle, visibility) MUST call requestUpdate()
+// before returning, or the viewport will go silently stale.
 class ViewportWindow : public QWindow {
     Q_OBJECT
 public:
@@ -217,8 +221,6 @@ private:
 
     QOpenGLContext* context_ = nullptr;
     QOpenGLFunctions_4_5_Core* gl_ = nullptr;
-    QTimer render_timer_;
-    QElapsedTimer frame_clock_;
     bool gl_initialized_ = false;
 
     // Shaders