Function eval_sibling_path 

pub fn eval_sibling_path(lesson_path: &Path) -> PathBuf

The eval-suite sibling of lesson_path: the same file name prefixed with EVAL_SIBLING_PREFIX, in the same directory.

Pure (§2.1): a path-to-path derivation with no filesystem access. This is THE convention — new scaffolds the sibling here, eval and eval-report discover it through cli’s delegating sibling_suite_path — so a suite authored (or scaffolded) next to its lesson is found by every consumer without configuration. A path with no file name (a bare root) yields the prefix alone, so a subsequent read fails with a path-named error rather than silently scoring nothing.