Payload, serialized by the publisher.
Entry kind. Only 'msg' exists today; readers must skip kinds they don't
know, so new kinds can be added without breaking older nodes.
Message id, unique and increasing per namespace.
Publish timestamp (ms), as given by the publisher.
A message as stored in a node's inbox.
endpointslists the endpoints of the reading node that the message is for: routing is resolved by the datastore, so the runtime never matches topics itself.