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/PlayerPose.cs

@@ -38,6 +38,14 @@ public class PlayerPose : ScriptableObject
     [Range(0.1f, 3f)]
     [SerializeField] private float runClipSpeed = 1f;
 
+    [Tooltip("Optional. Played instead of the idle clip while falling (airborne and descending past the " +
+             "controller's fall threshold). Leave empty to keep holding the idle pose while falling. Should loop.")]
+    [SerializeField] private AnimationClip fallClip;
+
+    [Tooltip("Playback speed multiplier for the fall clip. 1 = authored speed.")]
+    [Range(0.1f, 3f)]
+    [SerializeField] private float fallClipSpeed = 1f;
+
     [Header("Collision")]
     [Tooltip("CharacterController capsule center Y while this pose is active (meters, local space). Lower " +
              "this together with Height so a short pose still rests its feet on the floor.")]
@@ -82,6 +90,12 @@ public class PlayerPose : ScriptableObject
     /// <summary>Playback speed multiplier for the run clip (1 = authored speed).</summary>
     public float RunClipSpeed => runClipSpeed;
 
+    /// <summary>Optional clip played while falling; null = hold the idle pose.</summary>
+    public AnimationClip FallClip => fallClip;
+
+    /// <summary>Playback speed multiplier for the fall clip (1 = authored speed).</summary>
+    public float FallClipSpeed => fallClipSpeed;
+
     /// <summary>CharacterController capsule center Y applied while this pose is active.</summary>
     public float ControllerCenterY => controllerCenterY;