Function render_cap 

pub fn render_cap(template: &str, model: Option<&str>) -> String

A cap template rendered into a runnable command line (DESIGN.md §8). The model is bound exactly as a launch binds it — pasted in as the opaque name the operator configured, Voro being model-blind — and a template naming no model renders unchanged, which is what makes the per-model question optional rather than required.