Presence and client counts

When a socket joins events:<topic>, the server tracks presence metadata for that topic.

What you will observe

• Phoenix pushes a presence_state snapshot after a successful join.
• The JavaScript client emits a topicPresence event when it decodes a system envelope with event: "presence_change" for a subscribed topic.
• client.clientCount(topic) asks the channel for the current integer member count.

These mechanisms are intentionally small: you get occupancy signals without leaking unnecessary per-member data through the SDK unless you handle raw envelopes yourself.