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/specs/cutout-boundary-smoothing/spec.md

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+## ADDED Requirements
+
+### Requirement: Boundary-specific smoothing strength
+
+`O Image Depth Cutout` SHALL 使用单个 `Boundary Smooth` 输入控制边界平滑次数。Split 边 SHALL 使用完整 `Smooth Weight`，原始 Outline SHALL 使用 `0.1` 倍 `Smooth Weight`；两类影响相交时 Split 强度 MUST 优先。
+
+#### Scenario: Separate Split and Outline response
+
+- **WHEN** 同一网格同时存在 Split 边与原始 Outline 且启用 Boundary Smooth
+- **THEN** Split 边按完整 Smooth Weight 平滑
+- **AND** 仅受 Outline 影响的点按其 0.1 倍平滑
+
+#### Scenario: Split and Outline intersection
+
+- **WHEN** Split 与 Outline 的两圈影响范围相交
+- **THEN** 相交区域使用两者中较强的 Split influence
+
+### Requirement: Boundary Smooth default and bypass
+
+`O Image Depth Cutout` 的 `Boundary Smooth` SHALL 默认为 `4`，保持范围 `0–16`；值为 `0` 时 MUST 完全旁路边界平滑。
+
+#### Scenario: New node group defaults
+
+- **WHEN** 构建新的 O Image Depth Cutout 节点组
+- **THEN** Boundary Smooth 默认值为 4
+
+#### Scenario: Disabled boundary smoothing
+
+- **WHEN** Boundary Smooth 为 0
+- **THEN** Split 边和 Outline 均不发生边界平滑位移