Extract space generation algorithms to ifcopenshell.util

Move Blender-independent space generation algorithms from Bonsai
(GPL) to ifcopenshell.util (LGPL):

- ifcopenshell.util.shape.bisect_mesh_plane_vf: vectorized numpy
  triangle/plane intersection for mesh bisection
- ifcopenshell.util.element.iter_top_connections: walker for
  IfcRelConnectsElements(TOP) relationships
- ifcopenshell.util.space: new module with get_boundary_lines,
  get_space_polygon, get_auto_space_height and height detection
  helpers — all operating on IFC geometry without Blender

Bonsai's tool/spatial.py now delegates to these utilities via
thin wrappers, keeping only Blender-specific concerns (cache
management with depsgraph invalidation, UI property reads).

tool/wall.py iter_wall_slab_connections delegates to
ifcopenshell.util.element.iter_top_connections.

Added 22 tests: 6 for bisect_mesh_plane_vf, 10 for space
generation algorithms, 4 for iter_top_connections, 2 Bonsai
integration tests for cache behavior.

Generated with the assistance of an AI coding tool.

src/bonsai/bonsai/tool/wall.py

@@ -245,16 +245,9 @@ class Wall(bonsai.core.tool.Wall):
     @classmethod
     def iter_wall_slab_connections(cls, wall: ifcopenshell.entity_instance):
         """Yield ``(slab, rel)`` tuples for every ``IfcRelConnectsElements(TOP)``
-        connecting a slab to this wall — the rel kind ``extend_walls_to_underside``
-        creates. Walks ``wall.ConnectedFrom`` because the slab is the relating
-        side of the TOP rel."""
-        for rel in getattr(wall, "ConnectedFrom", []) or ():
-            if not rel.is_a("IfcRelConnectsElements") or rel.Description != "TOP":
-                continue
-            slab = rel.RelatingElement
-            if slab is None:
-                continue
-            yield slab, rel
+        connecting a slab to this wall. Delegates to
+        :func:`ifcopenshell.util.element.iter_top_connections`."""
+        yield from ifcopenshell.util.element.iter_top_connections(wall)
 
     @classmethod
     def iter_slab_wall_connections(cls, slab: ifcopenshell.entity_instance):