Add tests for IfcGridAxis fixes

- Add test_create_axis_curve.py with three unit tests: basic
  polyline creation, safe removal of an unshared existing curve,
  and preservation of a shared curve when only one referencing
  axis is updated (regression for the shallow-copy duplication bug)
- Add feature scenario "Export IFC - with duplicate-of-duplicate
  grid axis locations preserved" to project.feature, reproducing
  the case where duplicates of duplicates lost their positions on
  save/reload

src/bonsai/test/bim/feature/project.feature

@@ -920,6 +920,21 @@ Scenario: Export IFC - with moved grid axis location synchronised
     And I load previously saved IFC project
     Then the object "IfcGridAxis/01" bottom left corner is at "1,-2,0"
 
+Scenario: Export IFC - with duplicate-of-duplicate grid axis locations preserved
+    Given an empty IFC project
+    And I press "bim.add_grid"
+    And I set "scene.BIMGridProperties.is_locked" to "False"
+    And the object "IfcGridAxis/01" is selected
+    And I duplicate the selected objects
+    And the object "IfcGridAxis/01.001" is moved to "1,0,0"
+    And the object "IfcGridAxis/01.001" is selected
+    And I duplicate the selected objects
+    And the object "IfcGridAxis/01.002" is moved to "2,0,0"
+    When I save IFC project
+    And I load previously saved IFC project
+    Then the object "IfcGridAxis/01.001" bottom left corner is at "1,-2,0"
+    And the object "IfcGridAxis/01.002" bottom left corner is at "2,-2,0"
+
 Scenario: Export IFC - with changed object scale ignored
     Given an empty IFC project
     And I add a cube

src/ifcopenshell-python/test/api/grid/test_create_axis_curve.py

@@ -0,0 +1,88 @@
+# IfcOpenShell - IFC toolkit and geometry engine
+# Copyright (C) 2021 Dion Moult <dion@thinkmoult.com>
+#
+# This file is part of IfcOpenShell.
+#
+# IfcOpenShell is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Lesser General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# IfcOpenShell 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 Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with IfcOpenShell.  If not, see <http://www.gnu.org/licenses/>.
+
+import numpy as np
+
+import ifcopenshell.api.grid
+import test.bootstrap
+
+
+class TestCreateAxisCurve(test.bootstrap.IFC4):
+    def make_grid_with_axis(self, axis_tag="A"):
+        grid = self.file.createIfcGrid()
+        grid.ObjectPlacement = self.file.createIfcLocalPlacement(
+            RelativePlacement=self.file.createIfcAxis2Placement3D(
+                Location=self.file.createIfcCartesianPoint([0.0, 0.0, 0.0])
+            )
+        )
+        axis = ifcopenshell.api.grid.create_grid_axis(
+            self.file, axis_tag=axis_tag, same_sense=True, uvw_axes="UAxes", grid=grid
+        )
+        return grid, axis
+
+    def test_creates_a_polyline_axis_curve(self):
+        _, axis = self.make_grid_with_axis()
+        ifcopenshell.api.grid.create_axis_curve(
+            self.file, p1=np.array([0.0, 0.0, 0.0]), p2=np.array([10.0, 0.0, 0.0]), grid_axis=axis
+        )
+        assert axis.AxisCurve is not None
+        assert axis.AxisCurve.is_a("IfcPolyline")
+        assert len(axis.AxisCurve.Points) == 2
+
+    def test_replaces_existing_curve_when_unshared(self):
+        """Calling create_axis_curve again on the same axis replaces the old curve
+        and removes the old curve from the file when nothing else references it."""
+        _, axis = self.make_grid_with_axis()
+        ifcopenshell.api.grid.create_axis_curve(
+            self.file, p1=np.array([0.0, 0.0, 0.0]), p2=np.array([10.0, 0.0, 0.0]), grid_axis=axis
+        )
+        old_curve_id = axis.AxisCurve.id()
+        ifcopenshell.api.grid.create_axis_curve(
+            self.file, p1=np.array([1.0, 0.0, 0.0]), p2=np.array([11.0, 0.0, 0.0]), grid_axis=axis
+        )
+        assert axis.AxisCurve.id() != old_curve_id
+        assert self.file.by_id(old_curve_id) is None
+
+    def test_does_not_remove_shared_curve(self):
+        """When two axes share the same AxisCurve (e.g. after a shallow copy during
+        duplication), updating one axis must not destroy the curve still referenced
+        by the other axis."""
+        grid, axis = self.make_grid_with_axis()
+        axis2 = ifcopenshell.api.grid.create_grid_axis(
+            self.file, axis_tag="B", same_sense=True, uvw_axes="UAxes", grid=grid
+        )
+        ifcopenshell.api.grid.create_axis_curve(
+            self.file, p1=np.array([0.0, 0.0, 0.0]), p2=np.array([10.0, 0.0, 0.0]), grid_axis=axis
+        )
+        shared_curve = axis.AxisCurve
+        shared_curve_id = shared_curve.id()
+        # Simulate what copy_class produces: a duplicate axis that shares the
+        # source's AxisCurve rather than having its own copy.
+        axis2.AxisCurve = shared_curve
+        assert self.file.get_total_inverses(shared_curve) == 2
+
+        # Updating axis1's curve must not remove the curve that axis2 still needs.
+        ifcopenshell.api.grid.create_axis_curve(
+            self.file, p1=np.array([1.0, 0.0, 0.0]), p2=np.array([11.0, 0.0, 0.0]), grid_axis=axis
+        )
+        assert axis2.AxisCurve.id() == shared_curve_id
+        assert self.file.by_id(shared_curve_id) is not None
+
+
+class TestCreateAxisCurveIFC2X3(test.bootstrap.IFC2X3, TestCreateAxisCurve):
+    pass