ifcviewer: motion-adaptive contribution culling + sub-draw diagnostics

During camera motion, use a larger pixel-radius threshold (IFC_MIN_PX_MOTION)
to aggressively cull small objects, dramatically reducing sub_draws and
improving orbit fps (e.g. 29→67 fps on 1M-instance scene).  When the camera
stops, automatically re-cull at the base threshold to restore full detail.

Key behaviors:
- IFC_MIN_PX_MOTION=N sets the motion threshold (0 = disabled)
- Settle recull fires on the first still frame after motion
- HiZ pyramid invalidated on settle (stale from sparse motion frame)
- GPU cull results skipped on settle (dispatched at motion threshold)
- requestUpdate() ensures the settle frame actually runs

Also adds IFC_SUBDRAW_DIAG=1 diagnostic for sub-draw composition analysis
and documents Phase 3E/3F experiment results in README.

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

src/ifcviewer/ViewportWindow.h

@@ -376,6 +376,11 @@ private:
     QMatrix4x4 last_cull_proj_;
     bool       have_cached_cull_ = false;
 
+    // Motion-adaptive contribution culling.  During camera motion, use a
+    // larger pixel-radius threshold to aggressively cull small objects.
+    // When the camera stops, re-cull once at the base threshold.
+    bool       last_cull_was_motion_ = false;
+
     // Per-frame stats
     uint32_t visible_triangles_ = 0;
     uint32_t visible_objects_ = 0;