Introduce a new CirclePanel control that arranges visible children evenly around a circle. The control exposes Diameter, KeepVertical and OffsetAngle styled properties and implements MeasureOverride/ArrangeOverride to position and optionally rotate items on a circular path. Also add a demo UserControl (CirclePanelDemo.axaml and .axaml.cs) with a themed button template to showcase the panel.