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/Scripts/PlayerPoseController.cs

@@ -147,11 +147,12 @@ public partial class PlayerPoseController : MonoBehaviour
 
         PlayerPose pose = ActivePose();
 
-        // Pick the clip for the current locomotion state. Standing still, airborne, on a wall or on the
-        // ceiling holds the idle pose; moving on the ground plays the run clip while sprinting or the
-        // walk clip otherwise. Both locomotion clips are optional — a null one falls back to the idle
-        // pose, so an unset walk clip keeps the held-pose behaviour. PlayPose crossfades on any change
-        // and hands the rig back only when there is no clip at all (empty list / null idle clip).
+        // Pick the clip for the current locomotion state. Standing still, on a wall or on the ceiling
+        // holds the idle pose; moving on the ground plays the run clip while sprinting or the walk clip
+        // otherwise; falling (airborne and descending past the controller's threshold) plays the fall
+        // clip. All three extra clips are optional — a null one falls back to the idle pose, so an unset
+        // clip keeps the held-pose behaviour. PlayPose crossfades on any change and hands the rig back
+        // only when there is no clip at all (empty list / null idle clip).
         AnimationClip clip = pose ? pose.Clip : null;
         float speed = pose ? pose.ClipSpeed : 1f;
         if (pose && _player.IsMovingOnGround)
@@ -163,6 +164,11 @@ public partial class PlayerPoseController : MonoBehaviour
                 speed = _player.IsSprinting ? pose.RunClipSpeed : pose.WalkClipSpeed;
             }
         }
+        else if (pose && _player.IsFalling && pose.FallClip)
+        {
+            clip = pose.FallClip;
+            speed = pose.FallClipSpeed;
+        }
         PlayPose(clip, speed);
 
         // CharacterController capsule: each pose carries its own center Y / radius / height so a low pose