ifcviewer: scaffold ViewportHost + ViewportCore (Path A step 1)

Define the boundary the Path-A web-bring-up refactor will move things
across:

- ViewportHost.h is the embedder interface — surface creation,
  framebuffer geometry, frame scheduling, quit, and notification
  callbacks (onObjectPicked, onToolModeChanged, …). Desktop hosts
  forward notifications to Q_SIGNALS; the future web host pushes
  them to JS callbacks.

- ViewportCore.{h,cpp} is the platform-agnostic render-core target.
  Empty today — the body fills in across the #78-#86 sequence as
  each Qt subsystem (matrices, vectors, strings, timers, render
  path, input) gets de-Qt'd and moved over.

- ViewportWindow now multiply-inherits ViewportHost alongside QWindow
  and implements the host overrides as thin forwarders: createSurface
  returns the cached surface_, requestFrame -> requestUpdate, quit ->
  QCoreApplication::quit, onObjectPicked -> emit objectPicked.
  Renamed the DPR accessor `dpr()` (vs `devicePixelRatio`) to avoid
  the inherited-virtual clash with QWindow's qreal-returning version.

No method movement yet — this is purely the architectural scaffold so
subsequent commits have a destination.

src/ifcviewer/ViewportWindow.h

@@ -47,6 +47,8 @@
 #include "OverlayRenderer.h"
 #include "SelectionState.h"
 #include "StreamingThread.h"
+#include "ViewportCore.h"
+#include "ViewportHost.h"
 #include "VisibilityState.h"
 
 // Stage-2 wgpu viewport: opens a native QWindow, brings up a wgpu instance/
@@ -57,12 +59,37 @@
 //
 // Mirrors the lifecycle shape of the GL ViewportWindow so subsequent stages
 // can grow this into a full IFC renderer without restructuring the host.
-class ViewportWindow : public QWindow {
+//
+// Also implements ViewportHost: as the Path-A refactor moves rendering
+// state out into ViewportCore, this class plays the embedder role
+// (provides the wgpu surface, schedules frames, forwards notifications
+// to Q_SIGNALS). The web target's host is the analog on the Emscripten
+// side. Today most state still lives here; the override implementations
+// at the bottom of the class are the bridge for whatever has already
+// moved.
+class ViewportWindow : public QWindow, public ViewportHost {
     Q_OBJECT
 public:
     explicit ViewportWindow(QWindow* parent = nullptr);
     ~ViewportWindow();
 
+    // --- ViewportHost ----------------------------------------------------
+    //
+    // Implementations live in ViewportWindow.cpp alongside the
+    // platform-specific surface code so the Qt + native window-handle
+    // bits stay co-located. Notification overrides (onObjectPicked
+    // etc.) forward to the existing Q_SIGNALS so bonsai-side consumers
+    // see no change.
+    WGPUSurface createSurface(WGPUInstance instance) override;
+    void  framebufferSize(int& width_px, int& height_px) const override;
+    float dpr() const override;
+    void  requestFrame() override;
+    void  quit() override;
+    void  onObjectPicked(uint32_t object_id) override;
+    void  onSurfacePickedInTool(int x_px, int y_px, int modifiers) override;
+    void  onToolModeChanged(int tool_mode) override;
+    void  onToolBackspacePressed() override;
+
     void setBackgroundColor(const QColor& color);
 
     // Queue a sidecar path to be loaded after wgpu init completes. Safe to