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/Scenes/SampleScene.unity

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       value: Player
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-      propertyPath: m_IsActive
-      value: 1
-      objectReference: {fileID: 0}
-    - target: {fileID: 8001664246499114791, guid: cf153a3aea3d5da4f9d4cdbff0be8180, type: 3}
-      propertyPath: m_Enabled
-      value: 1
-      objectReference: {fileID: 0}
     m_RemovedComponents: []
     m_RemovedGameObjects: []
     m_AddedGameObjects: []