Scaffold experimental wgpu viewer backend

Adds src/ifcviewer-wgpu/ and src/ifcviewer-wgpu-minimal/ behind a new
BUILD_BONSAIVIEWER_WGPU option (default OFF), gated independently of
BUILD_BONSAIVIEWER. Stage 1 brings up a Qt window with a wgpu-native
v29 surface (X11) and clears to the background colour — no rendering
beyond that yet. Mirrors the lifecycle of the GL ViewportWindow so
subsequent stages (vertex-pulling renderer, pick, cull, HiZ, overlay)
slot in without restructuring the host.

wgpu-native is fetched as a pre-built binary release via FetchContent;
its .so SONAME is patched in at configure time so dependents get a
clean DT_NEEDED. The X11 native handle is obtained via the public
QNativeInterface::QX11Application API; Wayland and macOS/Windows
surface creation are stubbed with explicit "not wired yet" warnings.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ifcviewer-wgpu-minimal/main.cpp

@@ -0,0 +1,56 @@
+/********************************************************************************
+ *                                                                              *
+ * 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 <QApplication>
+#include <QCommandLineParser>
+#include <QMainWindow>
+#include <QWidget>
+#include <QVBoxLayout>
+
+#include "WgpuViewportWindow.h"
+
+// Stage-1 driver: opens a single window with the wgpu viewport embedded,
+// clears to background colour, and exits on close. The shape mirrors
+// ifcviewer-minimal so subsequent stages can grow this into a full
+// benchmark-comparable binary.
+int main(int argc, char* argv[]) {
+    QApplication app(argc, argv);
+    app.setApplicationName("IfcViewerWgpuMinimal");
+    app.setOrganizationName("IfcOpenShell");
+
+    QCommandLineParser parser;
+    parser.setApplicationDescription(
+        "IfcOpenShell minimal wgpu IFC viewer (stage 1: clear-color smoke test)");
+    parser.addHelpOption();
+    parser.process(app);
+
+    auto* viewport = new WgpuViewportWindow;
+    viewport->resize(1280, 800);
+
+    QWidget* container = QWidget::createWindowContainer(viewport);
+    container->setMinimumSize(320, 240);
+
+    QMainWindow main_window;
+    main_window.setWindowTitle("IfcViewer (wgpu) — stage 1");
+    main_window.setCentralWidget(container);
+    main_window.resize(1280, 800);
+    main_window.show();
+
+    return app.exec();
+}