A publish/subscribe transport carrying RealtimeFrames between
application instances.
publish(frame: RealtimeFrame): Promise<void>
Publishes a frame to every other subscribed instance.
The frame is not delivered back to this instance's own handlers — a local broadcast has already reached local members directly, and redelivering it would double-send.
subscribe(handler: RealtimeFrameHandler): Promise<() => void>
Registers a handler for frames arriving from other instances.
May be called more than once; each consumer plugin registers its own.
Reports whether the transport's backend is reachable right now, for the plugin's health indicator.
Optional: a transport with no meaningful liveness check omits it, and the indicator then reports only the lifecycle state.
This answers a fact (reachability), not a policy: the indicator that
consumes it owns the up/degraded/down mapping.