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.

此 issue 未指定檔案或測試；請先檢視描述中提到的 Processing PShape 參考與 Bevy Mesh 整合。完成條件是 Geometry 能夠作為 Bevy Mesh 載入，並提供生命週期、vertex-layout、CPU 端資料與 vertex-buffer 讀回 API。