Fix wall-split filled-opening classification + void copy

Two bugs in DumbWallJoiner.split's filled-opening branch:

1. Side classification read filling_obj.matrix_world.translation —
   flip-fragile because flip_object rotates the filler 180° + translates
   so the bbox stays visually in place, moving the door origin to the
   opposite bbox corner. A flipped door centred over the cut could be
   classified on the wrong side. Switch to the opening's axis-projected
   midpoint, which the unfilled-opening loop already uses.

2. When the void straddles the cut and the filling moves to element2,
   the void copy for element1 was taken from the rebound new_opening
   whose PlacementRelTo had been swapped to element2 — the new void on
   element1 then sat in element2's local frame. Reorder so the copy
   reads from the original opening (still hosted by element1) before
   remove_feature destroys it.

Generated with the assistance of an AI coding tool.

src/bonsai/test/bim/module/model/test_wall_split_filled_opening.py

@@ -0,0 +1,66 @@
+# Bonsai - OpenBIM Blender Add-on
+# Copyright (C) 2026
+#
+# This file is part of Bonsai.
+#
+# Bonsai is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# Bonsai is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with Bonsai.  If not, see <http://www.gnu.org/licenses/>.
+#
+# This file was generated with the assistance of an AI coding tool.
+
+"""Pins two contracts in ``DumbWallJoiner.split``'s filled-opening branch:
+
+1. Side classification reads the opening's axis-projected midpoint, not
+   the filling's ``matrix_world.translation``. The filling origin is
+   flip-fragile — ``flip_object`` rotates the filler 180° + translates so
+   the bbox stays visually in place, which would mis-classify a flipped
+   door centred over the cut.
+2. When the void straddles the cut and the filling moves to element2,
+   the void copy for element1 is taken from the ORIGINAL opening (whose
+   ``ObjectPlacement`` still references element1), not the rebound
+   ``new_opening`` (whose ``PlacementRelTo`` was swapped to element2)."""
+
+import inspect
+
+import pytest
+
+pytestmark = pytest.mark.wall
+
+
+def _split_source():
+    from bonsai.bim.module.model.wall import DumbWallJoiner
+
+    return inspect.getsource(DumbWallJoiner.split)
+
+
+def test_side_classification_uses_opening_midpoint_not_filling_origin():
+    source = _split_source()
+    assert "opening_midpoint" in source
+    # The pre-fix code projected the filling's world translation onto the
+    # axis to classify; that path must be gone.
+    assert "filling_obj.matrix_world.translation" not in source
+
+
+def test_void_copy_reads_from_original_opening_before_remove():
+    source = _split_source()
+    # Locate the "filling moves to element2" branch via the opening
+    # midpoint check; the void-copy and the trailing remove_feature both
+    # live inside this branch, after the prior unfilled-opening loops.
+    branch_start = source.index("if opening_midpoint > cut_percentage:")
+    branch = source[branch_start:]
+    add_idx = branch.index("_add_void_copy(element1, opening)")
+    remove_idx = branch.index("feature.remove_feature(tool.Ifc.get(), feature=opening)")
+    # Read-from-original is the whole point — the rebound ``new_opening``
+    # references element2's frame and would shift the void to element1's
+    # origin in element2's local coords.
+    assert add_idx < remove_idx