Add comprehensive tests for image processing and mask functionalities

- Introduced tests for color image handling, ensuring source preservation and premultiplied alpha usage.
- Implemented tests for brush stroke operations, validating segment boundaries and brush anti-aliasing.
- Added tests for frame anti-aliasing, confirming correct rendering behavior with various alpha values and depth settings.
- Created tests for frame projection, ensuring accurate overlap calculations and half-plane clipping.
- Developed tests for image premultiplied retention, verifying behavior during loading and packing processes.
- Added undo functionality tests for mask operations, ensuring single transaction behavior during edits.

docs/internals/node-assets.md

@@ -39,7 +39,7 @@ uv run build_node --skip-tests
 
 构建从空 Factory 文件生成全部节点组，关闭 `.blend1` 备份，并覆盖唯一发布资产。
 
-`O Image Cutout` 在四个 Shape 分支与平滑之后统一处理 **Inward Axis**。Shape 子节点保持内部 `+X` 朝向；默认 `-Z` 分支执行一次轴转换，`+X` 分支保留内部坐标。运行时对象矩阵与该接口配对，保证两种模式落在相同世界空间位置。
+四个 Cutout Shape 子节点统一接收局部 XY 平面，并以 `-Z` 表示向图片内部的方向。`O Image Cutout` 在 Shape 分支与平滑之后处理 **Inward Axis**：默认 `-Z` 直接输出原生几何，`+X` 分支执行一次 Z 到 X 的轴转换；运行时对象矩阵使用逆变换补偿，保证两种模式落在相同世界空间位置。
 
 `O Image Depth Plane`、`O Image Cutout`、`O Image Depth Balloon` 与 `O Image Depth Surface` 统一使用 **Reference Depth** 作为采样深度的运算基准。