Fix wall split: preserve door/window fill rel

Splitting a wall through a door orphaned the door (door.FillsVoids
became empty). The fill rel was being reassigned by setting its
RelatedBuildingElement slot — schema-wise that's the filling slot, not
the wall slot — so when remove_feature deleted the old opening it
also cascade-removed the rel. Transferring via RelatingOpeningElement
keeps the rel pointing at the new opening so the door stays
associated. Pre-existing bug from 5a6476a57, surfaced by ef144dce2.

Generated with the assistance of an AI coding tool.

src/bonsai/bonsai/bim/module/model/wall.py

@@ -1364,7 +1364,7 @@ class DumbWallJoiner:
                         tool.Ifc.get(), opening.Representation, exclude=["IfcGeometricRepresentationContext"]
                     )
 
-                rel.RelatedBuildingElement = element2
+                rel.RelatingOpeningElement = new_opening
 
                 # Remove the old opening
                 ifcopenshell.api.feature.remove_feature(tool.Ifc.get(), feature=opening)

src/bonsai/test/bim/feature/model.feature

@@ -285,6 +285,7 @@ Scenario: Split a wall which has a flipped door
     And the object "IfcWall/Wall" is selected
     And I press "bim.hotkey(hotkey='S_K')"
     Then the object "IfcDoor/Door" is at "8.01,0.1,0"
+    And the object "IfcWall/Wall.001" is filled by "IfcDoor/Door"
 
 Scenario: Offset walls
     Given an empty IFC project