s&box: Built-in PlayerController component — first/third person movement, jump, duck, IEvents callbacks

In s&box, the built-in PlayerController component provides a complete first/third-person player setup with physics, movement modes (walk, swim, climb, ladder), camera, animation, and footsteps. Developers build custom movement from scratch when PlayerController already handles most cases. It also exposes WishVelocity for AI integration.