Wires a Durable Object namespace up as the application's
IRealtimeBackplane under CAPABILITIES.REALTIME_BACKPLANE, so
WebSocket rooms and SSE channels reach clients on other replicas.
The namespace's class_name must be a Durable Object class the application
exports, delegating to RealtimeBackplaneObjectCore. No plugin
option can export that class on the application's behalf, which is why the
DO class is a documented deliverable rather than an implementation detail.
Registering this arm and RealtimeBackplanePlugin in one application
fails at startup: the kernel rejects two providers of one capability token.
Register exactly one.
binding: string
The Durable Object namespace binding name from wrangler.toml.
name: string
Instance name. 'default' (the default) claims the bare
realtime-backplane token; anything else derives
realtime-backplane.<name>.
topic: string
The object name every replica shares, passed to idFromName.
Two applications sharing one namespace must set different topics, or each receives the other's frames.