Function generate_shell 

pub fn generate_shell(
    verts: &[[f64; 3]],
    faces: &[[usize; 3]],
    thickness: f64,
) -> OffsetMesh

Generate a solid “shell” mesh from a surface mesh.

Creates an outer surface (offset by +thickness) and an inner surface (the original mesh), connected by side walls along the boundary. For closed meshes the result is a thick shell.