Function generate_shell 

pub fn generate_shell(
    inner_shell: &Mesh,
    params: &ShellParams,
) -> (Mesh, ShellResult)

Generate a printable shell from the inner surface.

Creates outer surface using the configured method (normal or SDF-based), then connects inner and outer at boundaries with a rim.

Arguments

• inner_shell - The inner surface mesh (from offset stage)
• params - Shell generation parameters

Returns

A tuple of (shell mesh, generation result).