refactor(actor): renamed Cell to Controller for clearer semantics

- Renamed Cell to Controller throughout actor module for clearer semantics.
- Renamed cell() methods to controller() and weak_cell() to weak_controller() across ActorRef and WeakActorRef traits.
- Added try_controller() method to WeakActorRef trait for upgrading weak references.
- Removed upgrade_cell() method from WeakAddr in favor of try_controller() on trait.