Function delivery_hold_signal 

fn delivery_hold_signal(domain: &DomainSession) -> bool

The turn signal a delivery is held on: only what a Result clears.

Stricter than turn_in_flight, which ORs in runtime_state for the in-flight guards - no Result clears that one, so a gate reading it could hold every later delivery with nothing left to release it. Both signals here are down once a Result has landed, which is what lets the hold and the turn-end drain pair up.