Function validate_mesh 

pub fn validate_mesh(
    vertices: &[[f64; 3]],
    triangles: &[[usize; 3]],
) -> Result<()>

Validate that a triangle mesh has consistent vertex/index arrays.

Checks:

1. vertices is non-empty.
2. All indices in triangles are less than vertices.len().
3. No triangle has repeated vertex indices (degenerate triangles).