Add WASD character controller with jump and slope sliding

- Add PlayerCharacterController script: CharacterController-based
  movement (camera-relative WASD), jump, gravity, rotate-toward-movement
  and slope sliding so the player slides off surfaces/edges steeper than
  the slope limit instead of balancing on them.
- Update SampleScene: add Player (with CharacterController), ground plane
  and obstacle cubes; parent the paintable paintman_UV under the Player.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Assets/Scripts/PlayerCharacterController.cs.meta

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