The epoch authenticator answers the question one member at a time, and the question is about the group: how many branches a run ended in, when each split off, and who is on it. Answering it means putting every member's log side by side, which stops being something to do by hand as soon as a group is more than a handful.
Read the epoch lines out of a run, group members by the authenticator they held at each epoch, and render the result as a lineage tree and one lane per member. Distinguish a member that forked from one that only fell behind, since both drop every frame the group sends and the drop count cannot tell them apart.
Take either one log per member or a single log split on its user_id spans, so the same report runs on the in-process group tests as on a farm of real peers.
