Function usg_cache_dir 

pub fn usg_cache_dir(unity_version: &str) -> String

Per-user cache root for tarball-extracted Unity packages. Subdirectory by Unity version isolates installs (each ships its own bundled tarballs). Honours XDG_CACHE_HOME; falls back to $HOME/.cache. Panics if neither is set — landing extracted Unity code in a world-writable /tmp would let anyone tamper with our compile inputs.