Add per-pose fall animation state and clip

Play a dedicated falling animation when the player is airborne and
descending past a scale-aware velocity threshold.

- PlayerCharacterController: new IsFalling state + Fall Animation Speed
  threshold (true only when !grounded && !wallClimbing && descending,
  scaled via CapsuleScale); explicitly false during wall/ceiling climb.
- PlayerPose: per-pose Fall Clip + Fall Clip Speed (mirrors walk/run).
- PlayerPoseController: select the fall clip while falling, fall back to
  the idle clip when a pose has none (no regression).
- Author content: Paintman-Fall.anim assigned to Pose_Idle; UMotion
  project, Player prefab and scene authoring.
- Docs: check off the fall animation in GAME_DESIGN.md.

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

Assets/Prefabs/Player.prefab

@@ -120,6 +120,7 @@ MonoBehaviour:
   jumpHeight: 1.2
   gravity: -14.715
   groundedStickVelocity: -2
+  fallAnimationSpeed: 0.1
   slideDownSteepSlopes: 1
   slideAcceleration: 20
   maxSlideSpeed: 8