Returns a specification to start this module under a supervisor.

See Supervisor.

@spec enqueue(LemonChannels.OutboundPayload.t()) ::
  {:ok, reference()} | {:error, term()}

Enqueue a payload for delivery.

If the content exceeds the channel's chunk limit, it will be automatically split into multiple messages.

Returns {:ok, ref} or {:error, reason}.

@spec stats() :: map()

Returns current outbox statistics.