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/ifcopenshell-python/test/util/test_element.py

@@ -1392,3 +1392,47 @@ class TestCopyDeepIFC4(test.bootstrap.IFC4):
         element2 = subject.copy_deep(self.file, element)
         assert element2.Segments[0][0] == (1, 2)
         assert element2.Segments[1][0] == (3, 4)
+
+
+class TestIterTopConnections(test.bootstrap.IFC4):
+    def test_yields_top_connected_element(self):
+        wall = ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcWall")
+        slab = ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcSlab")
+        rel = self.file.createIfcRelConnectsElements(
+            GlobalId=ifcopenshell.guid.new(),
+            RelatingElement=slab,
+            RelatedElement=wall,
+            Description="TOP",
+        )
+        results = list(subject.iter_top_connections(wall))
+        assert len(results) == 1
+        assert results[0][0] == slab
+        assert results[0][1] == rel
+
+    def test_returns_empty_when_no_connections(self):
+        wall = ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcWall")
+        assert list(subject.iter_top_connections(wall)) == []
+
+    def test_filters_non_top_description(self):
+        wall = ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcWall")
+        slab = ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcSlab")
+        self.file.createIfcRelConnectsElements(
+            GlobalId=ifcopenshell.guid.new(),
+            RelatingElement=slab,
+            RelatedElement=wall,
+            Description="BOTTOM",
+        )
+        assert list(subject.iter_top_connections(wall)) == []
+
+    def test_filters_non_rel_connects_elements(self):
+        wall = ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcWall")
+        slab = ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcSlab")
+        self.file.createIfcRelConnectsPathElements(
+            GlobalId=ifcopenshell.guid.new(),
+            RelatingElement=slab,
+            RelatedElement=wall,
+            Description="ATPATH",
+            RelatingConnectionType="ATPATH",
+            RelatedConnectionType="ATPATH",
+        )
+        assert list(subject.iter_top_connections(wall)) == []