Supervision tree for one named Docket runtime instance.

Hosts start one per runtime instance, usually through the use Docket host module, or directly:

children = [
  {Docket.Runtime.Supervisor,
   name: MyApp.DocketRuntime,
   backend: Docket.Postgres,
   repo: MyApp.Repo}
]

:name is required and becomes the runtime identity passed to durable facade functions. All other options are stored as the instance's instance configuration. Execution policy is resolved once at startup and cannot be replaced by per-call options. A configured backend: BackendModule contributes its own supervised child and is the only public durable backend substitution point; individual stores cannot be mixed.

A production instance always owns one backend bundle. The tree also owns a small instance-configuration process and a Task.Supervisor used by backend vehicles for node execution and observer delivery.