Proper naming for objects which have a position and bounds on a plane
Everything is an Entity!

If you would follow the Entity Component System approach, that is. The idea is that all objects are entities, and each Entity has several components. Size, Position, Movable - all this are different aspects of what an Entity possibly could do and are therefore each it's own Component. As for Systems, that's where most of the logic is.

Names like PositionableBoundable and MovableBoundable sound weird because it is weird, you are trying to use one name for two things. Composition over inheritance is the key here.

As for the name Boundable, I would probably have gone for just Box or Rect which are more well known.