Type Alias IdempotentStartDecision<TPersisted>

IdempotentStartDecision:
| { kind: "start"; reservation: StartReservation }
| { kind: "replay"; persisted: TPersisted; reservation: StartReservation }

What a surface must do next, once the reservation has been resolved.

{ kind: "start"; reservation: StartReservation }
- kind: "start"
  Nobody has started this key's run: proceed, using THIS runId.
- reservation: StartReservation
{ kind: "replay"; persisted: TPersisted; reservation: StartReservation }
- kind: "replay"
  The run already exists: answer with its persisted state, unchanged.
- persisted: TPersisted
- reservation: StartReservation