Function default_registry_dir 

pub fn default_registry_dir() -> PathBuf

The per-user artifact cache for this host — ${XDG_CACHE_HOME:-~/.cache}/chtypes/artifacts/<os>-<arch>, <arch> spelled the artifact way (amd64, arm64). Where scripts/fetch.sh and crate::ensure install, where a core-repository build lands, and what every SDK’s tests and playgrounds fall back to when $CHTYPES_REGISTRY is unset — one directory the four SDKs agree on. A path, not a promise: Registry::new still errors if nothing is there.