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.cpp

@@ -0,0 +1,82 @@
+/********************************************************************************
+ *                                                                              *
+ * 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 "SettingsWindow.h"
+#include "AppSettings.h"
+
+#include <QCheckBox>
+#include <QDialogButtonBox>
+#include <QFormLayout>
+#include <QLineEdit>
+#include <QShowEvent>
+#include <QVBoxLayout>
+
+SettingsWindow::SettingsWindow(QWidget *parent)
+    : QDialog(parent)
+{
+    setWindowTitle("Settings");
+    setupUi();
+}
+
+void SettingsWindow::setupUi() {
+    auto* form = new QFormLayout();
+
+    geometry_library_edit_ = new QLineEdit(this);
+    geometry_library_edit_->setMinimumWidth(280);
+    form->addRow("Geometry Library", geometry_library_edit_);
+
+    show_stats_check_ = new QCheckBox(this);
+    form->addRow("Show Performance Stats", show_stats_check_);
+
+    backface_culling_check_ = new QCheckBox(this);
+    backface_culling_check_->setToolTip(
+        "Skip triangles facing away from the camera.  Big FPS win on "
+        "closed solids; disable if you see holes in open geometry.");
+    form->addRow("Backface Culling", backface_culling_check_);
+
+    auto* button_box = new QDialogButtonBox(
+        QDialogButtonBox::Ok | QDialogButtonBox::Cancel, this);
+
+    auto* root = new QVBoxLayout(this);
+    root->addLayout(form);
+    root->addWidget(button_box);
+
+    connect(button_box, &QDialogButtonBox::accepted, this, &SettingsWindow::onAccepted);
+    connect(button_box, &QDialogButtonBox::rejected, this, &SettingsWindow::reject);
+}
+
+void SettingsWindow::showEvent(QShowEvent* event) {
+    // Re-sync widgets from the persisted settings every time the dialog is
+    // shown, so a previous Cancel doesn't leave stale text in the field.
+    syncFromSettings();
+    QDialog::showEvent(event);
+}
+
+void SettingsWindow::syncFromSettings() {
+    geometry_library_edit_->setText(AppSettings::instance().geometryLibrary());
+    show_stats_check_->setChecked(AppSettings::instance().showStats());
+    backface_culling_check_->setChecked(AppSettings::instance().backfaceCulling());
+}
+
+void SettingsWindow::onAccepted() {
+    AppSettings::instance().setGeometryLibrary(geometry_library_edit_->text());
+    AppSettings::instance().setShowStats(show_stats_check_->isChecked());
+    AppSettings::instance().setBackfaceCulling(backface_culling_check_->isChecked());
+    accept();
+}