spatial hierarchy from IFC + active-model concept

Build the spatial hierarchy panel from the loaded model's real IFC
spatial structure instead of mock data:
- helpers/element: add get_spatial_children (IsDecomposedBy -> RelatedObjects,
  filtered to spatial elements) to walk IfcProject -> IfcSite -> IfcBuilding
  -> IfcBuildingStorey -> IfcSpace.
- SessionState: relay dataSourceReady as modelDataSourceReady (the .ifc for a
  sidecar hit loads asynchronously, so the tree can only build once it arrives).
- spatial_hierarchy/View: walk the active model's IFC file into a TreeNode
  tree, naming nodes by Name (fallback to class), mapping site/building/storey
  kinds; siblings sorted with natural (numeric) collation.
- spatial_hierarchy/Panel: tree now fills the panel height (setBodyExpanding +
  Expanding size policy); right-click menu for recursive Expand/Collapse
  Subtree and Expand/Collapse All.

Add the concept of an active model:
- SessionState: activeModelId / setActiveModelId / activeModelChanged; the
  first loaded model is active by default; reassigns/clears on removal.
- Models panel: clicking a model makes it active; its cube icon is drawn with
  the accent colour (makeAccentSvgIcon) via FederationItemModel::setActiveModelId.
- The spatial hierarchy reflects only the active model.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/bonsaiviewer/modules/models/FederationItemModel.cpp

@@ -87,14 +87,28 @@ QStandardItem* FederationItemModel::makeGroupNameItem(const QString& group_id, c
     return item;
 }
 
+QIcon FederationItemModel::modelIcon(const QString& model_id) const {
+    return model_id == active_model_id_
+        ? components::icons::makeAccentSvgIcon(":/icons/cube.svg")
+        : components::icons::makeSvgIcon(":/icons/cube.svg");
+}
+
 QStandardItem* FederationItemModel::makeModelNameItem(const QString& model_id, const QString& display_name) const {
-    auto* item = new QStandardItem(components::icons::makeSvgIcon(":/icons/cube.svg"), display_name);
+    auto* item = new QStandardItem(modelIcon(model_id), display_name);
     item->setData(model_id, IdRole);
     item->setData(int(ItemKind::Model), KindRole);
     item->setEditable(false);
     return item;
 }
 
+void FederationItemModel::setActiveModelId(const QString& model_id) {
+    if (model_id == active_model_id_) return;
+    const QString previous = active_model_id_;
+    active_model_id_ = model_id;
+    if (auto* item = id_to_name_item_.value(previous)) item->setIcon(modelIcon(previous));
+    if (auto* item = id_to_name_item_.value(active_model_id_)) item->setIcon(modelIcon(active_model_id_));
+}
+
 QStandardItem* FederationItemModel::makeVisibilityItem(ItemKind kind, bool visible) const {
     QString icon_path;
     if (kind == ItemKind::Group) {

src/bonsaiviewer/modules/models/FederationItemModel.h

@@ -53,6 +53,10 @@ public:
     // preserving anyway).
     void rebuildAll();
 
+    // The active model is drawn with an accent-coloured cube icon. Restyles the
+    // previously- and newly-active model rows.
+    void setActiveModelId(const QString& model_id);
+
 private slots:
     void onGroupAdded(const QString& group_id);
     void onGroupRemoved(const QString& group_id);
@@ -77,8 +81,11 @@ private:
     void appendGroupSubtreeTo(QStandardItem* parent_item, const QString& group_id);
     void refreshSubtreeVisibility(QStandardItem* root);
 
+    QIcon modelIcon(const QString& model_id) const;  // accent cube when active, else plain
+
     Federation* federation_ = nullptr;
     QHash<QString, QStandardItem*> id_to_name_item_;  // both group_ids and model_ids
+    QString active_model_id_;
 };
 
 } // namespace bonsaiviewer::modules::models

src/bonsaiviewer/modules/models/Panel.cpp

@@ -249,8 +249,15 @@ ModelsPanel::ModelsPanel(bonsaiviewer::SessionState* session_state,
     addBodyWidget(section);
 
     connect(tree_, &QTreeView::clicked, this, [this](const QModelIndex& index) {
-        if (!index.isValid() || index.column() != 1) return;
-        commands::toggleVisibility(*session_state_, kindOf(index), idOf(index));
+        if (!index.isValid()) return;
+        if (index.column() == 1) {
+            commands::toggleVisibility(*session_state_, kindOf(index), idOf(index));
+            return;
+        }
+        // Clicking a model (its cube icon / row) makes it the active model.
+        if (kindOf(index) == ItemKind::Model) {
+            session_state_->setActiveModelId(idOf(index));
+        }
     });
 
     connect(tree_, &QTreeView::customContextMenuRequested, this, [this](const QPoint& pos) {

src/bonsaiviewer/modules/models/View.cpp

@@ -70,6 +70,9 @@ ModelsPanelView::ModelsPanelView(ModelsPanel* widget,
     connect(&bonsaiviewer::ViewerSettings::instance(),
             &bonsaiviewer::ViewerSettings::themeChanged,
             this, rebuild);
+    connect(session_state_, &SessionState::activeModelChanged, this, [this](const QString& model_id) {
+        model_->setActiveModelId(model_id);
+    });
 }
 
 } // namespace bonsaiviewer::modules::models