ifcviewer: move scene state into ViewportCore (#84-c)

Move the five scene-state fields that drive per-model GPU upload + the
streaming residency loop into ViewportCore:

  BufferPool      pool_              — vertex+index sub-allocator
  StreamingThread streaming_thread_  — background chunk reader
  std::unordered_map<uint32_t, ModelGpuData> models_gpu_
                                     — per-model state
  uint32_t        next_model_id_     — model-id allocator
  uint32_t        next_object_id_    — globally-unique object-id allocator

ViewportCore.h gains transitive includes for BufferPool / StreamingThread
/ ModelGpuData; ViewportWindow keeps the same names as reference aliases
so existing method bodies that touch them don't have to change.

Same risk profile as #84-a and #84-b: the storage moved but the values
are still set and consumed by the same code paths, so behaviour stays
identical.

Builds: desktop / bonsai / web all green. Tests 100/100.

src/ifcviewer/ViewportWindow.cpp

@@ -633,7 +633,12 @@ ViewportWindow::ViewportWindow(QWindow* parent)
       edge_bgl_             (core_.edge_bgl_),
       edge_pipeline_layout_ (core_.edge_pipeline_layout_),
       edge_pipeline_        (core_.edge_pipeline_),
-      pick_pipeline_(core_.pick_pipeline_) {
+      pick_pipeline_(core_.pick_pipeline_),
+      pool_           (core_.pool_),
+      streaming_thread_(core_.streaming_thread_),
+      models_gpu_     (core_.models_gpu_),
+      next_model_id_  (core_.next_model_id_),
+      next_object_id_ (core_.next_object_id_) {
     // wgpu doesn't need a GL context; we just need a real native window
     // whose backing layer matches the GPU API wgpu will drive.
     //