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/ViewportCore.cpp

@@ -0,0 +1,23 @@
+/********************************************************************************
+ *                                                                              *
+ * This file is part of IfcOpenShell.                                           *
+ *                                                                              *
+ * IfcOpenShell is free software: you can redistribute it and/or modify         *
+ * it under the terms of the Lesser GNU General Public License as published by  *
+ * the Free Software Foundation, either version 3.0 of the License, or          *
+ * (at your option) any later version.                                          *
+ *                                                                              *
+ * IfcOpenShell is distributed in the hope that it will be useful,              *
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of               *
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the                 *
+ * Lesser GNU General Public License for more details.                          *
+ *                                                                              *
+ * You should have received a copy of the Lesser GNU General Public License     *
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.         *
+ *                                                                              *
+ ********************************************************************************/
+
+#include "ViewportCore.h"
+
+ViewportCore::ViewportCore(ViewportHost* host) : host_(host) {}
+ViewportCore::~ViewportCore() = default;