ifcviewer: split into shared library and two app executables

Turn src/ifcviewer into libIfcViewer.so holding the rendering engine +
geometry pipeline (ViewportWindow, GeometryStreamer, BvhAccel,
InstancedGeometry, SidecarCache, LodBuilder, AppSettings).  Move the
existing UI shell (MainWindow, SettingsWindow, main.cpp) into
src/ifcviewer-full as the IfcViewerFull executable.  Add a new
src/ifcviewer-minimal target with a MinimalWindow that hosts only the
viewport and reuses the sidecar fast-path for benchmark/debug runs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/ifcviewer-minimal/main.cpp

@@ -0,0 +1,65 @@
+/********************************************************************************
+ *                                                                              *
+ * 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 <QSurfaceFormat>
+#include <QCommandLineParser>
+
+#include "MinimalWindow.h"
+
+int main(int argc, char* argv[]) {
+    QApplication app(argc, argv);
+    app.setApplicationName("IfcViewerMinimal");
+    app.setOrganizationName("IfcOpenShell");
+
+    QSurfaceFormat fmt;
+    fmt.setVersion(4, 5);
+    fmt.setProfile(QSurfaceFormat::CoreProfile);
+    fmt.setDepthBufferSize(24);
+    fmt.setSwapBehavior(QSurfaceFormat::DoubleBuffer);
+    fmt.setSamples(4);
+    QSurfaceFormat::setDefaultFormat(fmt);
+
+    QCommandLineParser parser;
+    parser.setApplicationDescription("IfcOpenShell minimal IFC viewer — benchmarking and debugging");
+    parser.addHelpOption();
+    parser.addPositionalArgument("files", "IFC file(s) to open", "[files...]");
+    parser.addOption({{"c", "camera"},
+        "Set camera: tx,ty,tz,dist,yaw,pitch", "params"});
+    parser.addOption({{"b", "benchmark"},
+        "Run N frames then print stats and exit", "frames"});
+    parser.process(app);
+
+    MinimalWindow window;
+    window.show();
+
+    auto args = parser.positionalArguments();
+    if (!args.isEmpty()) {
+        window.addFiles(args);
+    }
+
+    if (parser.isSet("camera")) {
+        window.setPendingCamera(parser.value("camera"));
+    }
+    if (parser.isSet("benchmark")) {
+        window.setPendingBenchmark(parser.value("benchmark").toInt());
+    }
+
+    return app.exec();
+}