Bonsai: populate empty derived set when usage-assigning material to typeless occurrence

Follow-up to the previous commit's material=None usage fix. When the
usage-path assignment runs on an occurrence that has no type, the API
derives a brand new empty IfcMaterialLayerSet (or IfcMaterialProfileSet)
and wraps it in the usage. core.assign_material then reads the assigned
material back with should_inherit=False, which returns the Usage entity
itself. The Usage fails is_a_material_set, so the add_material_to_set
follow-up that is supposed to seed the first layer/profile never runs
and the set stays empty with zero layers. The BDD scenario 'Edit layer
item defaults null IsVentilated to FALSE in UI' then dies with
IndexError on ifc.by_type('IfcMaterialLayer')[0].

Unwrap the usage to its ForLayerSet/ForProfileSet before the set
population step. add_material_to_set only seeds a set that is still
empty, so a populated layer set inherited from a type is untouched.

This change was made with the assistance of an AI tool.

src/bonsai/bonsai/core/material.py

@@ -150,12 +150,20 @@ def assign_material(
         assigned_material = material_tool.get_material(element)
         assert assigned_material  # Type checker.
 
+        # Usages wrap the actual material set, unwrap it so an empty derived
+        # set (occurrence without a type) still gets its first layer/profile.
+        assigned_material_set = assigned_material
+        if assigned_material.is_a("IfcMaterialLayerSetUsage"):
+            assigned_material_set = assigned_material.ForLayerSet
+        elif assigned_material.is_a("IfcMaterialProfileSetUsage"):
+            assigned_material_set = assigned_material.ForProfileSet
+
         if material_tool.is_a_material_set(material):
             # Ensure set is a valid IFC.
             default_material = material_tool.get_default_material()
             material_tool.add_material_to_set(material_set=material, material=default_material)
-        elif material_tool.is_a_material_set(assigned_material):
-            material_tool.add_material_to_set(material_set=assigned_material, material=material)
+        elif material_tool.is_a_material_set(assigned_material_set):
+            material_tool.add_material_to_set(material_set=assigned_material_set, material=material)
         material_tool.ensure_material_assigned(
             elements=[element], material_type=element_material_type, material=material
         )