Companion to nav/Fabric.Backend.Guild#<TBD> which adds the server-side
emitToUser broadcast on channel membership changes. Before, the inbound
only learned about new channels via the 60s polling resync (worst-case
60s lag). Now the backend tells us directly so sub/unsub is realtime.
socket.on('channel.joined', evt) → join the socket.io room for evt.channelId
and add to the local 'joined' set.
socket.on('channel.left', evt) → leave + remove from 'joined'.
Both events are idempotent (`if (joined.has(id))` / `if (!joined.has(id))`)
so duplicate emits from server are safe. Polling resync still runs every
60s as a safety net for transient socket drops between emit and
reconnect, partial server failures, etc.
When backend lacks this support (older deployments), nothing breaks —
the event simply never fires and polling carries the load as before.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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