Add tests and implement boundary smoothing and cutout functionality

- Introduced tests for boundary smoothing in `test_boundary_smoothing.py`, verifying that smoothing reduces stair-step artifacts while preserving boundary integrity.
- Added tests for cutout boundary behavior in `test_cutout_boundary.py`, ensuring proper handling of boundary triangles and normal blending.
- Created `test_temporary_attributes.py` to check the cleanup of temporary attributes across depth modifier branches, ensuring no warnings are raised.
- Implemented point culling functionality in `test_validity.py`, confirming that invalid points are removed without affecting adjacent geometry.
- Developed boundary smoothing logic in `boundary_smoothing.py`, allowing for depth smoothing while maintaining edge integrity.
- Added cutout boundary management in `cutout_boundary.py`, including functions for removing boundary triangles and tapering thickness profiles.
- Implemented validity checks in `validity.py` to ensure proper deletion of invalid vertices and associated wire edges.

tests/support/planes.py

@@ -63,6 +63,8 @@ def create_surface(source, plane_type):
                         ("Uniform Scale", 0.5), ("Reference Depth", 1.2),
                         ("Depth Scale", 0.0), ("Thickness", 0.0)):
         set_value(name, value)
+    if "Boundary Smooth" in inputs:
+        set_value("Boundary Smooth", 0)
     return obj, set_value, plane, inputs, image