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/helpers/element.cpp

@@ -159,6 +159,25 @@ express::Base get_aggregate_s(const express::Base& element) {
     return relationship.RelatingObject();
 }
 
+// The spatial-structure children aggregated under this element (IsDecomposedBy →
+// RelatedObjects, filtered to spatial elements).
+template <typename Schema>
+std::vector<express::Base> get_spatial_children_s(const express::Base& element) {
+    std::vector<express::Base> children;
+    const auto object = element.template as<typename Schema::IfcObjectDefinition>();
+    if (!object) {
+        return children;
+    }
+    for (const auto& relationship : object.IsDecomposedBy()) {
+        for (const auto& related : relationship.RelatedObjects()) {
+            if (related.template as<typename Schema::IfcSpatialStructureElement>()) {
+                children.push_back(related);
+            }
+        }
+    }
+    return children;
+}
+
 // ifcopenshell.util.element.get_container (should_get_direct=false, no
 // ifc_class): the directly containing spatial element, or the container of the
 // aggregate parent for an aggregated part.
@@ -239,6 +258,20 @@ express::Base get_container(const express::Base& element) {
     unsupported_schema(name);
 }
 
+std::vector<express::Base> get_spatial_children(const express::Base& element) {
+    if (!element) {
+        return {};
+    }
+    const std::string name = schema_name(element);
+#define IFCOPENSHELL_DISPATCH(Schema, Identifier)       \
+    if (name == Identifier) {                           \
+        return get_spatial_children_s<Schema>(element); \
+    }
+    IFCOPENSHELL_HELPER_FOR_EACH_SCHEMA(IFCOPENSHELL_DISPATCH)
+#undef IFCOPENSHELL_DISPATCH
+    unsupported_schema(name);
+}
+
 // getattr(element, name) for a string- or enum-valued attribute. Reads by name,
 // so it works uniformly across the various subtypes that carry a given
 // attribute (PredefinedType, Name, ...).