Cluster fan-out adapter for modern MCP subscriptions.
The adapter keeps registrations in a configurable storage adapter on the
node that owns each listener and distributes untargeted publications over a
Phoenix.PubSub-compatible module. No Phoenix dependency is required by
ExMCP; applications that select this adapter must provide a module exporting
subscribe/2 and broadcast_from/4.
Publications constrained to a transport_ref remain node-local because a
transport process is owned by exactly one registry.