ifcviewer: draw 3D pivot indicator during navigation

A small RGB axis cross is rendered at camera_target_ while the user is
orbiting, panning, or has just zoomed.  Visibility toggles on
middle-mouse press/release; the wheel arms a single-shot QTimer that
hides it 750 ms after the last notch.

Drawn in two passes: GL_GREATER at 30% alpha for the occluded portion
(X-ray cue) and GL_LEQUAL at full alpha for the visible portion.  Arm
length is computed from camera_distance_, fovy, and viewport height so
the cross stays ~30 px on screen across zoom levels.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ifcviewer/ViewportWindow.h

@@ -28,6 +28,10 @@
 #include <QVector3D>
 #include <QSet>
 
+QT_BEGIN_NAMESPACE
+class QTimer;
+QT_END_NAMESPACE
+
 #include <vector>
 #include <unordered_map>
 #include <cstdint>
@@ -231,9 +235,16 @@ private:
     void render();
     void renderPickPass();
     void renderAxisGizmo();
+    void renderPivotIndicator();
     void updateCamera();
     void buildShaders();
     void buildAxisGizmo();
+    void buildPivotIndicator();
+    // Show/hide the orbit-pivot marker.  hide_after_ms > 0 starts a single-shot
+    // timer that auto-hides — used by the wheel handler to give the marker a
+    // short afterglow after zoom.  Drag-based callers pass 0 and toggle
+    // visibility manually on press/release.
+    void setPivotIndicatorVisible(bool visible, int hide_after_ms = 0);
     void setupVaoLayout(GLuint vao, GLuint vbo, GLuint ebo);
 
     // Resolve the default framebuffer's MSAA depth into a single-sample
@@ -315,6 +326,14 @@ private:
     GLuint axis_vao_ = 0;
     GLuint axis_vbo_ = 0;
 
+    // Orbit-pivot indicator: 1 center vertex + (N+1) rim vertices on the unit
+    // circle (last == first to close the triangle fan).  Rendered as a screen-
+    // space disc at camera_target_, visible only while the user is navigating.
+    GLuint pivot_program_ = 0;
+    GLuint pivot_vao_ = 0;
+    GLuint pivot_vbo_ = 0;
+    int    pivot_rim_count_ = 0;
+
     // Per-model GPU data
     std::unordered_map<uint32_t, ModelGpuData> models_gpu_;
 
@@ -411,6 +430,11 @@ private:
     Qt::MouseButton active_button_ = Qt::NoButton;
     QPoint last_mouse_pos_;
 
+    // Pivot indicator visibility — true while drag-navigating, or briefly
+    // after a wheel notch (the timer auto-clears it).
+    bool    pivot_indicator_visible_ = false;
+    QTimer* pivot_indicator_hide_timer_ = nullptr;
+
     // FPS/fly mode state.  fps_keys_held_ tracks WASD/QE/Shift between
     // press+release; fps_last_tick_ gates dt inside render().
     // fps_ignore_next_mouse_move_ swallows the synthetic MouseMove that