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/frame.md

@@ -0,0 +1,21 @@
+# Frame 多图合成
+
+`operators/image_edit_tool/frame.py` 定义 `FrameImages`、`FrameTool` 和工具激活入口。Frame 将选中的静态 Image Empty 按当前视图合成为一张图片。
+
+```mermaid
+flowchart TD
+    A[拖出屏幕矩形] --> B[冻结视图矩阵并收集选中静态图片]
+    B --> C[检查矩形与 active 图片的有效投射交集]
+    C --> D[按 Region 与 Maximum Frame Resolution 计算输出尺寸]
+    D --> E[观察射线投影到各图片平面]
+    E --> F[按样本深度从远到近合成，等深度 active 优先]
+    F --> G[边界覆盖采样与独立 Alpha]
+    G --> H[创建 Packed Image 并复用 active Object]
+    H --> I[删除其他参与图片对象并提交 Undo]
+```
+
+正交视图接受有限深度，透视视图采样观察方向前方的交点。交叠检查采用连续投影范围；输出尺寸受 Region、最长边上限和总像素上限约束。
+
+RGB 与 Alpha 分别累计，保留被 Mask 隐藏的有效颜色。边界使用覆盖采样，内部复用直接采样，避免黑边和不必要的模糊。结果平面朝向观察者，位置由 active 图片的有效深度确定。
+
+Frame 保留 active 对象身份、名称和 Collection；图像替换复用 [Common](../common.md) 的共享数据与失败恢复逻辑。