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-full/SettingsWindow.h

@@ -0,0 +1,49 @@
+/********************************************************************************
+ *                                                                              *
+ * 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/>.         *
+ *                                                                              *
+ ********************************************************************************/
+
+#ifndef SETTINGSWINDOW_H
+#define SETTINGSWINDOW_H
+
+#include <QDialog>
+
+class QCheckBox;
+class QLineEdit;
+class QShowEvent;
+
+class SettingsWindow : public QDialog {
+    Q_OBJECT
+public:
+    explicit SettingsWindow(QWidget *parent = nullptr);
+
+protected:
+    void showEvent(QShowEvent* event) override;
+
+private slots:
+    void onAccepted();
+
+private:
+    void setupUi();
+    void syncFromSettings();
+
+    QLineEdit* geometry_library_edit_ = nullptr;
+    QCheckBox* show_stats_check_ = nullptr;
+    QCheckBox* backface_culling_check_ = nullptr;
+};
+
+#endif