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/Poses/Pose_Idle.asset

@@ -18,4 +18,10 @@ MonoBehaviour:
   walkClipSpeed: 1.4
   runClip: {fileID: -203655887218126122, guid: a4d4588e12cac6f48a17583c08a100d2, type: 3}
   runClipSpeed: 1.5
-  ceilingYOffset: 0
+  fallClip: {fileID: 7400000, guid: 460c2fe161ff3a6449648312c4e14eff, type: 2}
+  fallClipSpeed: 1
+  controllerCenterY: 0.66
+  controllerRadius: 0.1
+  controllerHeight: 1.3
+  cameraTargetYOffset: 0
+  firstPersonCameraTargetYOffset: 0