feat(node_assets): add image plane geometry nodes and validation scripts

- Implemented `image_plane.py` to build the Image Plane Geometry Nodes group with customizable parameters for geometry, subdivide, and thickness.
- Created `image_plane_validation.py` to validate the functionality of the image plane, ensuring correct geometry generation and material assignment.
- Developed `material_layers.py` to construct shader node groups for image materials, including depth and normal handling.
- Authored `plane_layouts.py` to define node arrangements for image geometry assets.
- Introduced `shadeless.py` to create a shadeless shader node group that displays color without scene lighting while preserving shadows and reflections.

docs/internals/operators/rectify.md

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+# Rectify 透视校正
+
+`operators/image_edit_tool/rectify.py` 定义 `RectifyImagePerspective` 与 `RectifyTool`，将图片内的四角区域校正为矩形。
+
+```mermaid
+flowchart TD
+    A[点选四角] --> B[规范为凸 Quad 并检查图片交叠]
+    B --> C[512×512 预览与宽高比调整]
+    C --> D[按像素密度和输出上限确定尺寸]
+    D --> E[分块 Homography 与预乘 Alpha 插值]
+    E --> F[按可见 Alpha 裁剪]
+    F --> G[映射 placement bounds 并替换 Packed Image]
+```
+
+四角允许任意点击顺序，退化或非凸输入在提交前校验。预览根据源颜色空间配置显示，像素重采样复用 `common/image.py`。输出最长边上限 8192，总像素上限 16,777,216。
+
+`replace_empty_image()` 根据校正区域的 placement bounds 调整画框，保留源 Image 名称、对象身份和共享引用语义。