feat: Implement weight cutout boundary smoothing for image depth cutout

- Added a new proposal for boundary smoothing in the weight cutout process, differentiating between Split edges and Outline smoothing requirements.
- Introduced a new capability `cutout-boundary-smoothing` to control smoothing strength and default values.
- Updated specifications to define the behavior of boundary-specific smoothing strength and default settings.
- Created tasks to verify the implementation of boundary behavior and asset updates.
- Developed comprehensive tests for depth symmetry and normal scaling to ensure expected behavior and performance.
- Introduced new attributes for normal mapping to facilitate persistent face attributes sharing between depth symmetry and image materials.
- Built a new node group for O Image Depth Symmetry, integrating the new smoothing capabilities and ensuring compatibility with existing workflows.

openspec/changes/weight-cutout-boundary-smoothing/tasks.md

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+## 1. Boundary behavior
+
+- [x] 1.1 区分 Depth Cutout 的 Split 与 Outline influence，并在单一平滑循环中应用完整与 0.1 倍权重。
+- [x] 1.2 将 O Image Depth Cutout 的 Boundary Smooth 默认值改为 4，保持零值旁路和既有范围。
+
+## 2. Verification and asset
+
+- [x] 2.1 更新相关行为与接口测试，覆盖 Split 优先、Outline 弱权重、默认值和关闭行为。
+- [x] 2.2 运行相关测试，执行 `uv run node-group build` 并确认保存资产验证通过。