feat: Implement depth cutout symmetry and associated functionalities

- Introduced a new proposal for depth cutout symmetry, allowing for the reuse of depth replacement and geometry operations.
- Added requirements for maintaining default thickness values for cutout nodes and ensuring proper initialization during object creation.
- Enhanced symmetry node to handle existing geometry, including alignment, mirroring, and boundary filling.
- Implemented fill smoothing to control the geometry of generated walls while preserving original surfaces.
- Created tests to validate the functionality of the new symmetry features against baseline geometry.
- Updated existing assets and node groups to support the new symmetry capabilities without introducing additional dependencies.

openspec/changes/compose-depth-cutout-symmetry/specs/shell-normal-smoothing/spec.md

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+## ADDED Requirements
+
+### Requirement: 薄壳法线平滑不受相对厚度衰减
+
+Shell 的 Normal Smooth SHALL 控制原正面点域法线的模糊次数，归一化后的结果用于壳体厚度偏移方向。系统 SHALL 不使用 Shell Thickness / Reference Depth 衰减该平滑；厚度控制偏移距离，Balloon 保留现有权重行为。
+
+#### Scenario: 小厚度下调节平滑
+- **WHEN** 非平面且法线有起伏的正面以小厚度生成 Shell，用户增大 Normal Smooth
+- **THEN** 壳体的归一化偏移方向发生相应平滑变化，且不会因厚度与参考深度的比值接近零而退回原始法线
+- **AND** 保持前表面相同而改变厚度时，平滑方向保持一致、偏移距离随厚度变化
+
+### Requirement: 法线平滑保留正面与零厚度行为
+
+Shell 的 Normal Smooth SHALL 仅通过厚度偏移作用于壳体，保留原深度正面；零厚度 SHALL 不因该参数改变几何。
+
+#### Scenario: 零厚度与平面
+- **WHEN** Shell 厚度为 0 时改变 Normal Smooth
+- **THEN** 顶点位置与面连接保持一致
+- **AND** 对常法线平面使用非零厚度时，改变 Normal Smooth 也不改变壳体形状