Wire generated pose icons, icon scales and pose button sprite

- Icon_pose_{ball,hold,idle,sit,star,wall}.png (rendered via PoseIconStage) as sprites
- Pose assets: icon references + per-pose iconScale tuning; Hold re-serialized with missing default fields
- Main: pose overview button sprite; Pose Icon scene framing tweaks; Player.mat shader switch

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Assets/Scenes/Main.unity

@@ -7209,7 +7209,7 @@ PrefabInstance:
     - target: {fileID: 6262868416683193056, guid: 2f104ea08ab5c4141b58109d91dbe7df, type: 3}
       propertyPath: m_Sprite
       value: 
-      objectReference: {fileID: 0}
+      objectReference: {fileID: 21300000, guid: cebd8b979b47a9144a018ec3d07b2b18, type: 3}
     - target: {fileID: 8162620711324250373, guid: 2f104ea08ab5c4141b58109d91dbe7df, type: 3}
       propertyPath: m_Name
       value: Icon Button - Pose