feat: distributed presence and state coordination

[catgoose]

Split from #142.

Presence tracking (OnJoin/OnLeave, subscriber counts) is currently process-local. In multi-instance deployments, presence state is incomplete — each instance only knows about its own subscribers.

Scope

• Shared presence state across instances (likely via the backend interface)
• Accurate subscriber counts cluster-wide
• Coordinated OnFirst/OnLast hooks that fire based on global state, not per-instance state

Depends on

• Production backend adapters (durable replay backends issue)