Package paths computes user flows — "what do users do after event X?" — the
Flows / Pathfinder / Paths report. For each user it anchors at their first
occurrence of the start event and follows the next steps in time order, then
aggregates the ranked next-event at each depth. Deterministic.
After follows up to `depth` steps after each user's first `start` event and
ranks what they did at each step (a user who stops contributes an implicit drop).