A relate page counts facts, declared relations, coordinate relations and
tensions alike; facts carry their text, so the page is kept smaller than
a trace page.
Whether a relation is the one that may cross an about: same_event_as
or same_entity_as, evidential, with why and evidence, stamped as
declared from a kmp_relate proposal, to a ref this about does not own.
Entry ids are strict descendants of the about they mutate. The about
anchor itself is deliberately excluded: ingesting an entry must never
change the root node’s kind or payload.