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.

AGENTS.md

@@ -96,4 +96,4 @@ uv run sync_models
 - 修改器 Geometry Nodes 的输入 socket 默认使用 `SINGLE`
 - socket `subtype` 按业务语义设置：长度用 `DISTANCE`，`0–1` 比例用 `FACTOR`，`0–100` 用 `PERCENTAGE`；无对应范围语义时保留默认
 - 接口或布局变更须验证 socket 顺序与结构、未使用输出隐藏及几何求值；纯布局变更比较运算、默认值和实际连接，确认功能不变
-- 在 Blender 节点编辑器按实际绘制尺寸检查总览和局部步骤，确认主轴、支线长度、节点间距及配对区域清楚；坐标检查不能代替视觉检查
+- 在 Blender 节点编辑器按实际绘制尺寸检查总览和局部步骤，确认主轴、支线长度、节点间距及配对区域清楚；