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Barriers a running factory is holding, and what happens to a flight that meets one.
§Why flights are parked and processes are not
A joined agent needs several inputs together. The obvious implementation — start it and let it block until the rest arrive — costs a live process per waiting branch, and those processes are agent CLIs: they have context windows, they cost money per minute in some pricing models, and a supervisor that dies leaves them orphaned. Parking the flight costs a map entry.
It also makes the wait durable. A parked flight is data, so it survives being written down; a blocked process is not.
§Why a dead barrier is abandoned rather than left
A barrier waiting for an upstream that no live run can still produce will wait forever, holding work somebody asked for. Silent permanent stalling is the worst outcome in this system — worse than a failure, which at least says something happened. So after every pass the barriers are checked against what can still be reached, and the ones that cannot complete are given up and reported.