ifcviewer-full: add .ifcfed federation save/load

Federation (JSON) tracks an ordered list of model sources plus an
optional home-view camera state. Sources are stored relative when
under the federation file's directory, absolute otherwise.

File menu now exposes New / Open / Save / Save As; Add Files moves
to Ctrl+Shift+O. View menu gains Set/Go to Home View. Window title
binds to dirty state via setWindowModified, and the close-window
prompt offers Save/Discard/Cancel.

Per-model transform (4x4 column-major) and visible round-trip
through load/save but are not yet applied at the viewport — the
georeferencing work uses them.

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

src/ifcviewer-full/MainWindow.h

@@ -34,6 +34,7 @@
 #include "ViewportWindow.h"
 #include "SceneLoader.h"
 
+class Federation;
 class SettingsWindow;
 
 class MainWindow : public QMainWindow {
@@ -43,12 +44,22 @@ public:
     ~MainWindow();
 
     void addFiles(const QStringList& paths);
+    bool openFederation(const QString& path);
     void setPendingCamera(const QString& params);
     void setPendingBenchmark(int frames);
 
+protected:
+    void closeEvent(QCloseEvent* event) override;
+
 private slots:
     void onFileOpen();
     void onDatabaseOpen();
+    void onFederationNew();
+    void onFederationOpen();
+    bool onFederationSave();
+    bool onFederationSaveAs();
+    void onSetHomeView();
+    void onGoHomeView();
     void onFileSettings();
     void onObjectPicked(uint32_t object_id);
     void onTreeSelectionChanged();
@@ -69,6 +80,11 @@ private slots:
 private:
     void setupUi();
     void setupMenus();
+    void loadModelsFromPaths(const QStringList& paths,
+                             const QStringList& fed_ids);
+    void clearScene();
+    bool confirmDiscardIfDirty();
+    void updateWindowTitle();
     void populateProperties(uint32_t object_id);
     void appendElementToTree(uint32_t model_id,
                              uint32_t object_id,
@@ -84,6 +100,7 @@ private:
 
     ViewportWindow* viewport_ = nullptr;
     SceneLoader*    loader_   = nullptr;
+    Federation*     federation_ = nullptr;
     SettingsWindow* settings_ = nullptr;
     QWidget* viewport_container_ = nullptr;
     QTreeWidget* element_tree_ = nullptr;
@@ -95,6 +112,11 @@ private:
     // Per-model tree roots, keyed by model_id.
     std::map<uint32_t, QTreeWidgetItem*> tree_roots_;
 
+    // Bidirectional federation_id <-> model_id map.  Federation owns the
+    // persistent ids; SceneLoader owns the runtime model_ids.
+    std::unordered_map<QString, uint32_t> fed_id_to_model_id_;
+    std::unordered_map<uint32_t, QString> model_id_to_fed_id_;
+
     // Display-side element registry for tree + property lookup.
     std::unordered_map<uint32_t, ElementInfo> element_map_;
     std::unordered_map<uint32_t, QTreeWidgetItem*> tree_items_;