Registry::append_family / load_family_members write+read a fixed-name
families.jsonl sibling via self.path.with_file_name(...), so isolation between
registries is per-directory, not per-filename — two Registry::open calls with
different runs filenames in one directory share a single family store. The
sibling was documented on append_family but not on Registry::open, where a caller
binds the path; this records the per-directory-isolation consequence at the bind
point. Pure rustdoc ratify of existing (cycle 0045) behaviour.
closes#82
Brummel
03f80f0a95