Top-level supervisor for PhiaUI collaboration infrastructure.

Starts the RoomRegistry (a Registry for unique room name lookups) and RoomSupervisor (a DynamicSupervisor for on-demand room processes).

Add to your application supervision tree only if using collab features:

# In MyApp.Application
children = [
  # ... your other children ...
  PhiaUi.Collab.Supervisor
]

The strategy is :one_for_all because the DynamicSupervisor depends on the Registry for name resolution — if either crashes both must restart.