ifcviewer: move wgpu lifecycle state ownership into ViewportCore

First chunk of the #84 ViewportCore extraction. The seven wgpu lifecycle
handles (instance, adapter, device, queue, surface, surface_format,
surface_configured) now live as ViewportCore members; ViewportWindow
keeps reference aliases pointing at ViewportCore's storage so its
existing render-method bodies don't need a `core_.` prefix added at
every call site — 230+ touches deferred until each method moves
across.

Member init order in ViewportWindow's constructor:
  core_(this)               → constructs ViewportCore with host_=this
  instance_(core_.instance_) → binds the alias to core_'s field
  …                           same for adapter/device/queue/surface/…

Friend declaration on ViewportCore::ViewportWindow lets the references
bind to its private fields. The friend bond shrinks each commit as
render methods (and their `device_` / `queue_` references) migrate into
ViewportCore proper; the goal state is no friend and no aliases.

Next #84 chunks (separate commits) move pipelines, models_gpu_, pool_,
streaming_thread_, then the render/cull/encode methods. Each leaves
the desktop build green.

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

src/ifcviewer/ViewportWindow.cpp

@@ -606,7 +606,19 @@ static WGPUStringView svFromCStr(const char* s) {
 // -----------------------------------------------------------------------------
 
 ViewportWindow::ViewportWindow(QWindow* parent)
-    : QWindow(parent) {
+    : QWindow(parent),
+      core_(this),
+      // Bind reference aliases to ViewportCore's storage so the
+      // existing `device_` / `queue_` / … sites in this TU keep
+      // working unchanged. Each reference goes away as its owning
+      // render method moves into ViewportCore.
+      instance_          (core_.instance_),
+      adapter_           (core_.adapter_),
+      device_            (core_.device_),
+      queue_             (core_.queue_),
+      surface_           (core_.surface_),
+      surface_format_    (core_.surface_format_),
+      surface_configured_(core_.surface_configured_) {
     // wgpu doesn't need a GL context; we just need a real native window
     // whose backing layer matches the GPU API wgpu will drive.
     //