Function default_source_cache_dir 

pub fn default_source_cache_dir() -> PathBuf

PM.6/PM.7b: the git source cache — ~/.config/lattice/cache/sources/.

A cache, not config: a deleted checkout is re-cloned, so it belongs in <config-home>/lattice/cache/sources/ rather than beside the user’s plugin.tomls. Falls back to the temp dir when no config home resolves, which keeps the resolver working rather than failing.