Comparison token. Implicit tokens carry a zattrs: prefix so they can
never collide with a producer-stamped content hash.
archive-etag: a zipped store, identified by a HEAD on the archive
(its root attrs are inside the file, so no document can be re-read).
Remote validation token for a dataset: either the producer-stamped
content_hashattr, or — when the dataset lacks one — an implicit token derived from the raw root.zattrsbytes (zattrs-hashmode).