DynamicSupervisor for flow runner processes.

Host applications start this supervisor in their own tree (usually under the application supervisor):

children = [
  # ... other children ...
  Caravela.Flow.Supervisor
]
Supervisor.start_link(children, strategy: :one_for_one)

Once running, Caravela.Flow.start/3 starts runners as supervised children. If the supervisor is not running, start/3 falls back to starting an unsupervised runner — handy for tests and tooling.