Function packs_cache_dir 

pub fn packs_cache_dir() -> PathBuf

Root for .harnpack archives unpacked by harn run <bundle.harnpack>. Each verified bundle is replayed into <root>/<sanitized-bundle-hash>/ so re-runs reuse the unpacked tree. Honors $HARN_CACHE_DIR/packs when set, otherwise XDG / $HOME/.cache/harn/packs.