Agent.presenceRequests keeps a p: null entry for every presence that has left #764

Summary

Agent._broadcastPresence stores every presence that gets through receivePresence middleware in presenceRequests[channel][id], and that includes p: null leaves. Entries are only dropped when the client unsubscribes from the channel or disconnects. So a long-lived connection that creates and destroys presences with fresh ids, such as one LocalPresence per doc with a generated id, keeps a dead entry for every id until it disconnects.

The null entries do no work. Since #751, the leave broadcast skips them. Their only remaining job is the pv comparison, which stops an older presence that finishes middleware later from being stored over the leave.

Suggested fix

When a presence leaves, drop its entry, and the channel's map once it's empty, instead of storing it. Dropping the entry also loses its pv, which #763 needs in order to tell that a late presence is stale. So keep the entry only while a broadcast for the same id is still in flight, and drop it once that broadcast finishes.