'ephemeral': a flow WITHOUT .persistent() checkpointed only so another replica can
continue it (handover.flows: 'forward'); 'stub': a marker of a conversation that
cannot be forwarded (an async bot.conversation), read by handover.fallback.
Not resumable after this instant (epoch ms); expired entries are discarded.
OptionalshapeEphemeral: fingerprint of the flow's steps, to detect a flow changed by a deploy.
OptionalnameConversation name (stubs; the flow id for ephemeral checkpoints).
OptionalstartedWhen the conversation started (epoch ms).
OptionalstartInitial c.state, when it was plain JSON (fallback: 'restart' re-enters with it).
See FlowCheckpoint.handover.