Function lesson_template 

pub fn lesson_template(language: Language, id: &str) -> String

Render a starter lesson for language under the slug id.

Pure (§2.1): it picks the language-appropriate exercise body and frames it with the lesson’s lesson_name, language, and a description — returning the YAML as data with no filesystem access, so the result is asserted directly against the production parser. The language choice drives the template, so a --lang python lesson never declares language: R (§1.2). The caller is responsible for id being a safe slug; add_lesson guards it.