Server-side entry point for pushing playback JS to every connected overlay.
Used by mix demo_director.play to broadcast a saved demo's JS
to whichever browser tab(s) currently have the overlay loaded.
Summary
Functions
@spec play!(String.t()) :: :ok
Broadcasts the given JS string to every joined overlay.
Reads the host app's pubsub server name from
Application.fetch_env!(:demo_director, :pubsub). Raises if the
pubsub server has not been configured (the install task or manual
wiring should set it).