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.

openspec/changes/update-pasted-brush-preview/proposal.md

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+## Why
+
+Texture Paint 模式粘贴图片后，笔刷已使用该图片作为纹理，但图标仍显示原有预览，难以直接识别绘制内容。将笔刷图标同步为粘贴图片，让选择笔刷时的视觉提示与实际纹理一致。
+
+## What Changes
+
+- Texture Paint 模式绑定粘贴图片后，同步更新当前可编辑笔刷的自定义预览。
+- 预览保持图片比例，支持已打包的剪贴板图片及重复粘贴更新。
+- 验证预览即时刷新，以及保存并重新打开文件后的保留效果。
+
+## Capabilities
+
+### New Capabilities
+
+- `pasted-brush-preview`: Texture Paint 粘贴图片与笔刷预览同步。
+
+### Modified Capabilities
+
+## Impact
+
+- `src/anyimage/operators/clipboard_image/actions.py`：笔刷纹理绑定与预览更新。
+- `src/anyimage/common/image.py`：复用现有图片读取或导出能力。
+- `tests/anyimage/operators/test_clipboard_image.py`：增加预览行为覆盖。
+- 使用 Blender 自定义预览 API，沿用现有依赖。