pub fn read_lesson_file(path: &Path) -> Result<Lesson, LoadError>Expand description
Read and parse a lesson from a file on disk.
The thin effectful shell over the pure Lesson::parse (§2.2): it performs
the file read, then delegates all structure and validation to parse. The
two failure modes stay distinct in the type so a read error is never
mistaken for a validation error (§3.1).