Function default_cache_dir 

pub fn default_cache_dir(library: EndfLibrary) -> PathBuf

Compute the default on-disk cache directory for a given library without constructing an EndfRetriever.

The retriever’s constructor builds a reqwest blocking client + TLS configuration, which is wasted work when all the caller needs is the cache path for a UI hint or manual drop instruction. Mirrors the path layout that EndfRetriever::new would resolve to.