Instanced Rendering

* **Description**: Efficiently render many copies of the same mesh (e.g., trees, grass, crowds, particles) using GPU instancing to reduce draw calls.
* **Acceptance Criteria**:
* Must support `glDrawElementsInstanced` for rendering multiple instances in a single draw call
* Must provide an instance buffer to store per-instance data (transforms, colors, etc.)
* Must allow users to specify instance count and per-instance attributes
* Must integrate with the Material System to pass instance data to shaders
* Must provide factory methods or helpers for common use cases (particle systems, foliage)