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/anyimage/operators/test_brush_preview_integration.py

@@ -0,0 +1,37 @@
+import bpy
+import numpy as np
+
+from anyimage.operators.clipboard_image.actions import _set_brush_preview
+
+
+def test_packed_brush_preview_preserves_image_and_survives_reopen(tmp_path):
+    bpy.ops.wm.read_factory_settings(use_empty=True)
+    image = bpy.data.images.new("Preview Source", width=80, height=40, alpha=True)
+    pixels = np.zeros((40, 80, 4), dtype=np.float32)
+    pixels[:, :40] = (1.0, 0.0, 0.0, 1.0)
+    image.pixels.foreach_set(pixels.ravel())
+    image.pack()
+    original_pixels = tuple(image.pixels)
+    original_format = image.file_format
+    brush = bpy.data.brushes.new("Preview Brush", mode="TEXTURE_PAINT")
+    brush.asset_mark()
+    try:
+        _set_brush_preview(bpy.context, brush, image)
+        assert tuple(brush.preview.image_size) == (80, 40)
+        preview = np.array(brush.preview.image_pixels_float[:]).reshape(40, 80, 4)
+        assert np.allclose(preview[:, :40, 0], 1.0, atol=1 / 255)
+        assert np.all(preview[:, :40, 3] == 1.0)
+        assert np.all(preview[:, 40:, 3] == 0.0)
+        assert tuple(image.pixels) == original_pixels
+        assert tuple(image.size) == (80, 40)
+        assert image.packed_file is not None
+        assert image.file_format == original_format
+        expected_preview = tuple(brush.preview.image_pixels)
+        path = str(tmp_path / "brush-preview.blend")
+        bpy.ops.wm.save_as_mainfile(filepath=path)
+        bpy.ops.wm.open_mainfile(filepath=path)
+        restored = bpy.data.brushes["Preview Brush"]
+        assert tuple(restored.preview.image_size) == (80, 40)
+        assert tuple(restored.preview.image_pixels) == expected_preview
+    finally:
+        bpy.ops.wm.read_factory_settings(use_empty=True)