Delivers an event to one member of a Realtime app, addressing the person rather than a channel. Every connection that member currently holds receives it, across tabs and devices, so there is no need to track their connections or give them a channel of their own. The member must have signed in on the connection for it to be addressable. Delivery is best-effort and not queued: a member holding no connections at the moment of the call simply does not receive the event. The event name clients bind to. Application event names are free-form; the `bird:` and `bird_internal:` prefixes are reserved for the protocol and rejected. Arbitrary JSON payload delivered as the event data — an object, array, or scalar. Cap: 10 KB serialized.