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/refine-cutout-split-boundaries/proposal.md

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+## Why
+
+Depth Cutout 的 Split 新边界仍继承原 Balloon 厚度，容易膨胀成尖脊；Boundary Smooth 在锯齿三角形上可能翻面，进一步改变厚度方向。恐龙头骨实图实验已确认边界厚度收敛、切口圆润和薄厚度下减弱法向平滑的效果，需要将这些行为整理为正式节点实现。
+
+## What Changes
+
+- Split 后清理三个顶点都位于边界的三角面，降低锯齿切口的退化风险。
+- 新切口的 Balloon 厚度在边界收敛为零，邻近区域逐渐恢复；Edge Round 同时控制新切口的圆润过渡。
+- 法向平滑结果按厚度连续混合：薄厚度接近原法向，正常厚度恢复既有平滑效果，默认始终启用。
+- 保留 Split Threshold、Boundary Smooth 及厚度、Edge Round 的实时调节，参数变化后重新计算边界与形状。
+
+## Capabilities
+
+### New Capabilities
+
+- `cutout-split-boundary-shaping`：切口清理、边界厚度收敛、圆润过渡及厚度关联法向平滑。
+
+### Modified Capabilities
+
+## Impact
+
+- `tools/nodes/groups/image_depth_cutout.py` 及必要的 `tools/nodes/common` 构建函数。
+- `tests/tools/nodes` 的 Cutout、边界平滑及属性生命周期测试。
+- `src/anyimage/assets/O_AnyImage.blend` 中的 Cutout 节点资产及对应加载版本。
+- 基于当前工作区的边界深度平滑继续实施，保留已有修改。