Implement `Geometry` API Object

We need to implement that basis for PShape. Let's steal the name from p5 and call this Geometry. Shape is kind of a weird term. Really, this is vertex data and we typically don't refer to 3d models as "shapes."

In Processing4, Shape contains Phong material info in its vertex layout. We definitely don't want to do this because we want to support materials as a first class API.

Steps:

• Make it loadable as a Bevy Mesh.
• Add lifecycle methods.
• Build APIs for interacting with vertex layout, defining vertex attributes, etc.
• Add support for modifying CPU side vertex data.
• Add vertex buffer readback support. This is going to be a bit tricky as Bevy packs multiple meshes into vertex buffer slabs.

该 issue 未指定文件或测试；请先查看描述中提到的 Processing PShape 参考和 Bevy Mesh 集成。完成标准是 Geometry 能够作为 Bevy Mesh 加载，并提供生命周期、vertex-layout、CPU 端数据和 vertex-buffer 读回 API。