Add new node asset scripts for geometry and shader operations

- Implemented `build_mesh_plane.py` to create a geometry node group for mesh planes with customizable parameters.
- Created `build_shadeless.py` for a shader node group that displays color without scene lighting while preserving shadows and reflections.
- Added `validate.py` to validate the integrity of the newly created node groups and their inputs.
- Introduced `validate_cutout.py` to ensure the functionality of cutout node groups and their dependencies.
- Developed `validate_mesh_plane.py` to test the mesh plane creation and its modifier settings.

docs/internals/operators/remove-background.md

@@ -0,0 +1,31 @@
+# Remove Background
+
+本页对应 `src/anyimage/operators/remove_background.py`。该模块负责从 Image Empty 准备媒体输入、提交 BEN2 去背任务，并用结果 Image 替换当前图像；模型推理留在 Server 进程。
+
+## 对应文件
+
+```text
+src/anyimage/
+├── operators/
+│   ├── remove_background.py       # Blender 入口、任务参数与结果替换
+│   └── common/image.py            # 媒体输入准备与结果 Image 创建
+└── server/
+    ├── jobs/remove_background.py  # 单图与多帧任务编排
+    └── models/ben2.py             # BEN2 前景适配
+```
+
+## 主流程
+
+```mermaid
+flowchart TD
+    Empty["Image Empty"] --> Prepare["准备图片、视频或序列输入"]
+    Prepare --> Operator["RunBackgroundRemoval"]
+    Operator --> Job["remove-background Job"]
+    Job --> Frames["生成 PNG 结果序列"]
+    Frames --> Import["创建结果 Image"]
+    Import --> Replace["替换 Empty.data"]
+```
+
+Packed 图片或内存中修改过的单图会先导出为临时输入；文件图片、视频和序列则沿用可读取的源路径。多帧输入由统一媒体层展开，结果保持连续帧命名，导入后继续沿用源 Image User 的帧起点、偏移与循环设置。
+
+BEN2 推理前使用输入 Alpha 屏蔽透明区域，模型输出的前景 Alpha 再与源 Alpha 合成，RGB 保持为源图内容。响应阶段只替换当前 Empty 的 Image 数据；当旧 Image 没有其他用户和 Fake User 时，旧数据块会被清理并由结果继承名称。