Delivers a message that arrived from another replica to this replica's local channel members.

Called only by the plugin's backplane subscription. It uses the registry's local-only delivery path, so an arriving message is never re-published — which would echo it around the cluster forever.

Frames of another kind, and frames this instance published itself, are ignored: one backplane topic carries both SSE channels and WebSocket rooms, and a channel may legitimately share a name with a room.