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/proposal.md

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+## Why
+
+`O Image Depth Cutout` 的 Split 边与原始 Outline 对几何平滑的需求不同：Split 边需要完整强度消除切缝折痕，而 Outline 只需轻微整理，避免轮廓被过度软化。
+
+## What Changes
+
+- `Boundary Smooth` 继续作为唯一的边界平滑次数控制，默认值改为 `4`。
+- Split 边使用完整 `Smooth Weight`，Outline 使用其 `0.1` 倍；两类影响相交时由 Split 强度优先。
+- 保持 `Boundary Smooth = 0` 完全关闭边界平滑。
+
+## Capabilities
+
+### New Capabilities
+
+- `cutout-boundary-smoothing`: 定义 Depth Cutout 对 Split 边与 Outline 的差异化平滑强度和默认控制值。
+
+### Modified Capabilities
+
+无。
+
+## Impact
+
+影响 `tools/nodes/common/boundary_smoothing.py`、`tools/nodes/groups/image_depth_cutout.py`、相关节点测试和 `src/anyimage/assets/O_AnyImage.blend`；不增加或移除公开输入。