Function read 

pub fn read(
    cache_root: &Path,
    repo_path: &Path,
    key: &str,
) -> Option<CachedNarrative>

Read the cached narrative at key, or None when it is absent or corrupt.

A missing file is the ordinary miss and is silent. A file that exists but no longer deserializes into a CachedNarrative — a truncated write, an older on-disk shape — is logged at warn and treated as a miss, so a corrupt sidecar can neither crash a run nor be served as if it were valid.