Delivers a frame that arrived from another replica to this replica's local room members.

Called only by the plugin's backplane subscription. It uses the room registry's local-only delivery path, so an arriving frame 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 WebSocket rooms and SSE channels, and a room may legitimately share a name with a channel.