Create a proper raw mesh example (#1957)

[joelreymont]

Python implementation:

• Replaced GLTF/trimesh loading with procedural primitive generation
• Added functions to generate: cube, pyramid, sphere, icosahedron, grid
• Removed trimesh and requests dependencies
• Removed download_dataset.py (no longer needed)
• Updated README to reflect procedural generation approach
• Added command-line options for sphere subdivisions and grid size

Rust implementation:

• Replaced GLTF parsing with procedural primitive generation
• Added MeshPrimitive struct with Clone derive
• Implemented same geometric primitives as Python version
• Removed gltf and bytes dependencies
• Updated README with new usage instructions
• Added command-line options matching Python version

Geometric primitives demonstrate:

• Colored Cube: per-vertex colors
• Textured Pyramid: UV coordinates with checkerboard texture
• Smooth Sphere: vertex normals for smooth shading
• Icosahedron: flat shading (no normals)
• Grid instances: transform hierarchy with translation, rotation, scale

Closes #1957

[grtlr]

Overall I think these changes are valuable, because they make the example more self-contained, so thank you very much for opening a PR!

There are a couple of things I think need to change though:

• Create a single mesh and then re-use that with a different subsets of parameters (albedo_factor, albedo_texture, …) to reduce the amount of geometry code in the example.
• I'm not really sure what the benefits of the Grid Meshes are, they use the same Transform3D archetype as the other logging calls. That being said, I don't think we need to cram instancing into this example.

[joelreymont]

Please check!

[grtlr]

@rerun-bot full-check

[grtlr]

Your recent force pushes seem to have overwritten the requested changed that you previously committed.