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.cpp

@@ -25,6 +25,7 @@
 #include "StreamingLoader.h"
 #include "VertexQuantization.h"
 
+#include <QCoreApplication>
 #include <QGuiApplication>
 #include <QResizeEvent>
 #include <QDebug>
@@ -628,6 +629,57 @@ ViewportWindow::~ViewportWindow() {
     shutdown();
 }
 
+// ---- ViewportHost overrides ------------------------------------------------
+//
+// Scaffolding for Path-A. ViewportCore is empty today, so these don't
+// yet have callers; the abstract methods exist only to define the
+// boundary that subsequent commits will rely on. Each notification
+// forwards to the existing Q_SIGNAL so bonsai-side consumers see no
+// change.
+
+WGPUSurface ViewportWindow::createSurface(WGPUInstance /*instance*/) {
+    // initWgpu() still drives surface creation through the private
+    // no-arg createSurface() helper that populates surface_. Once
+    // ViewportCore takes over the wgpu init flow it'll call this
+    // override instead and the private helper goes away; for now the
+    // override is a getter.
+    return surface_;
+}
+
+void ViewportWindow::framebufferSize(int& width_px, int& height_px) const {
+    const float r = float(QWindow::devicePixelRatio());
+    width_px  = int(QWindow::width()  * r);
+    height_px = int(QWindow::height() * r);
+}
+
+float ViewportWindow::dpr() const {
+    return float(QWindow::devicePixelRatio());
+}
+
+void ViewportWindow::requestFrame() {
+    requestUpdate();
+}
+
+void ViewportWindow::quit() {
+    QCoreApplication::quit();
+}
+
+void ViewportWindow::onObjectPicked(uint32_t object_id) {
+    emit objectPicked(object_id);
+}
+
+void ViewportWindow::onSurfacePickedInTool(int x_px, int y_px, int modifiers) {
+    emit surfacePickedInTool(x_px, y_px, modifiers);
+}
+
+void ViewportWindow::onToolModeChanged(int tool_mode) {
+    emit toolModeChanged(static_cast<ToolMode>(tool_mode));
+}
+
+void ViewportWindow::onToolBackspacePressed() {
+    emit toolBackspacePressed();
+}
+
 void ViewportWindow::setBackgroundColor(const QColor& color) {
     background_color_ = color;
     if (isExposed()) requestUpdate();