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/SessionState.h

@@ -78,6 +78,12 @@ public:
     QString modelIdForSessionModelId(uint32_t session_model_id) const;
     QList<uint32_t> sessionModelIds() const;
 
+    // The active model — the single model the spatial hierarchy (and other
+    // model-scoped views) operate on. Set by clicking a model in the models
+    // panel; defaults to the first loaded model. Empty when no model is loaded.
+    QString activeModelId() const { return active_model_id_; }
+    void setActiveModelId(const QString& model_id);
+
     void notifySelectionChanged();
     void notifyModelsChanged();
     void notifyFederationChanged();
@@ -101,6 +107,12 @@ signals:
     // for both sidecar-cache and stream loads; subscribers that just need to
     // re-derive view state (e.g. ViewportView::refresh) listen to this.
     void modelGeometryReady(uint32_t session_model_id);
+    // Fires when a model's live IFC data source (the .ifc/.rdb, opened in the
+    // background after a sidecar-cache hit) becomes available for queries —
+    // e.g. so the spatial hierarchy can be built once the file is loaded.
+    void modelDataSourceReady(uint32_t session_model_id);
+    // Fires when the active model changes (empty model_id when cleared).
+    void activeModelChanged(const QString& model_id);
     // Fires when SceneLoader reports a load failure. SessionState turns the
     // raw loader signal into a session-level one so views (e.g. the MessageBox)
     // can subscribe without touching the loader directly.
@@ -121,6 +133,7 @@ private:
     QString status_detail_;
     QHash<QString, uint32_t> model_id_to_session_model_id_;
     QHash<uint32_t, QString> session_model_id_to_model_id_;
+    QString active_model_id_;
     QHash<QString, QVariantMap> cloud_metadata_;
 };