ifcviewer: move scene mutators + releaseWgpuModelGpuData into ViewportCore (#84-f)

Move the eight scene-mutation methods that drive bonsai's load/unload
and georeference setters, plus the per-model GPU teardown helper.
All are mechanical transplants — no logic change — so behaviour stays
identical; only the owner has changed.

Methods moved (ViewportWindow public-API methods stay as forwarders
to keep the bonsai-side callers compiling):
  removeModel / resetScene / hideModel / showModel
  setFederatedFalseOrigin
  setModelCoordinateOperation
  setModelTransformation
  recomposeAndUploadModel

State moved:
  bool wgpu_initialized_   (storage → core_, alias kept in VW for
                            the initWgpu call site that still flips
                            it; goes when initWgpu moves)

Free function moved:
  releaseWgpuModelGpuData(ModelGpuData&, BufferPool&) → ViewportCore.cpp
  (must live in IfcViewerCore now that ViewportCore.cpp's
   removeModel / resetScene call it; ViewportWindow.cpp's remaining
   two call sites continue to resolve through ModelGpuData.h's
   declaration — same linker view, different definition TU)

The `if (isExposed()) requestUpdate()` Qt pattern inside the moved
bodies became `host_->requestFrame()` since ViewportCore can't see
QWindow; the desktop ViewportHost override at the bottom of
ViewportWindow.cpp continues to translate that into requestUpdate().

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

src/ifcviewer/ViewportWindow.h

@@ -634,7 +634,7 @@ private:
     // is unknown, has no instances, or wgpu init hasn't completed.
     void recomposeAndUploadModel(uint32_t model_id);
 
-    bool wgpu_initialized_ = false;
+    bool& wgpu_initialized_;
     int  configured_w_ = 0;
     int  configured_h_ = 0;