feat: Add CirclePanel control and demo.

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.

src/Avalonia/HandyControlDemo_Avalonia/UserControl/Styles/CirclePanelDemo.axaml

@@ -0,0 +1,49 @@
+<UserControl xmlns="https://github.com/avaloniaui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:hc="https://handyorg.github.io/handycontrol"
+             x:Class="HandyControlDemo.UserControl.CirclePanelDemo">
+    <UserControl.Resources>
+        <ControlTheme x:Key="CirclePanelButton" TargetType="Button">
+            <Setter Property="UseLayoutRounding" Value="False" />
+            <Setter Property="Height" Value="77" />
+            <Setter Property="Width" Value="36.06" />
+            <Setter Property="Background" Value="Transparent" />
+            <Setter Property="BorderThickness" Value="0" />
+            <Setter Property="Padding" Value="0" />
+            <Setter Property="Template">
+                <ControlTemplate TargetType="Button">
+                    <Panel Width="36.06" Height="77">
+                        <Path Data="{StaticResource CirclePanelDemoGeometry}"
+                              Fill="{DynamicResource PrimaryBrush}"
+                              Stretch="Fill" />
+                        <Path Data="{StaticResource CirclePanelRightGeometry}"
+                              Fill="White"
+                              Stretch="Uniform"
+                              Height="21"
+                              HorizontalAlignment="Center"
+                              VerticalAlignment="Center" />
+                    </Panel>
+                </ControlTemplate>
+            </Setter>
+            <Style Selector="^:pointerover">
+                <Setter Property="Opacity" Value=".9" />
+            </Style>
+            <Style Selector="^:pressed">
+                <Setter Property="Opacity" Value=".6" />
+            </Style>
+            <Style Selector="^:disabled">
+                <Setter Property="Opacity" Value="0.4" />
+            </Style>
+        </ControlTheme>
+    </UserControl.Resources>
+    <hc:CirclePanel Margin="64" Diameter="170">
+        <Button Theme="{StaticResource CirclePanelButton}" />
+        <Button Theme="{StaticResource CirclePanelButton}" />
+        <Button Theme="{StaticResource CirclePanelButton}" />
+        <Button Theme="{StaticResource CirclePanelButton}" />
+        <Button Theme="{StaticResource CirclePanelButton}" />
+        <Button Theme="{StaticResource CirclePanelButton}" />
+        <Button Theme="{StaticResource CirclePanelButton}" />
+        <Button Theme="{StaticResource CirclePanelButton}" />
+    </hc:CirclePanel>
+</UserControl>

src/Avalonia/HandyControlDemo_Avalonia/UserControl/Styles/CirclePanelDemo.axaml.cs

@@ -0,0 +1,9 @@
+namespace HandyControlDemo.UserControl;
+
+public partial class CirclePanelDemo : Avalonia.Controls.UserControl
+{
+    public CirclePanelDemo()
+    {
+        InitializeComponent();
+    }
+}

src/Avalonia/HandyControl_Avalonia/Controls/Panel/CirclePanel.cs

@@ -0,0 +1,100 @@
+using System;
+using System.Linq;
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Layout;
+using Avalonia.Media;
+
+namespace HandyControl.Controls;
+
+public class CirclePanel : Panel
+{
+    public static readonly StyledProperty<double> DiameterProperty =
+        AvaloniaProperty.Register<CirclePanel, double>(nameof(Diameter), defaultValue: 170.0);
+
+    public double Diameter
+    {
+        get => GetValue(DiameterProperty);
+        set => SetValue(DiameterProperty, value);
+    }
+
+    public static readonly StyledProperty<bool> KeepVerticalProperty =
+        AvaloniaProperty.Register<CirclePanel, bool>(nameof(KeepVertical));
+
+    public bool KeepVertical
+    {
+        get => GetValue(KeepVerticalProperty);
+        set => SetValue(KeepVerticalProperty, value);
+    }
+
+    public static readonly StyledProperty<double> OffsetAngleProperty =
+        AvaloniaProperty.Register<CirclePanel, double>(nameof(OffsetAngle), defaultValue: 0d);
+
+    public double OffsetAngle
+    {
+        get => GetValue(OffsetAngleProperty);
+        set => SetValue(OffsetAngleProperty, value);
+    }
+
+    static CirclePanel()
+    {
+        AffectsMeasure<CirclePanel>(DiameterProperty, KeepVerticalProperty, OffsetAngleProperty);
+    }
+
+    protected override Size MeasureOverride(Size availableSize)
+    {
+        var diameter = Diameter;
+        var layoutChildren = Children.Where(element => element.IsVisible).ToList();
+
+        if (layoutChildren.Count == 0) return new Size(diameter, diameter);
+
+        var newSize = new Size(diameter, diameter);
+
+        foreach (var element in layoutChildren)
+        {
+            element.Measure(newSize);
+        }
+
+        return newSize;
+    }
+
+    protected override Size ArrangeOverride(Size finalSize)
+    {
+        var keepVertical = KeepVertical;
+        var offsetAngle = OffsetAngle;
+        var layoutChildren = Children.Where(element => element.IsVisible).ToList();
+
+        if (layoutChildren.Count == 0)
+        {
+            return finalSize;
+        }
+
+        var i = 0;
+        var perDeg = 360.0 / layoutChildren.Count;
+        var radius = Diameter / 2;
+
+        foreach (var element in layoutChildren)
+        {
+            var centerX = element.DesiredSize.Width / 2.0;
+            var centerY = element.DesiredSize.Height / 2.0;
+            var angle = perDeg * i++ + offsetAngle;
+
+            element.RenderTransformOrigin = new RelativePoint(centerX, centerY, RelativeUnit.Absolute);
+            element.RenderTransform = new RotateTransform
+            {
+                Angle = keepVertical ? 0 : angle
+            };
+
+            var r = Math.PI * angle / 180.0;
+            var x = radius * Math.Cos(r);
+            var y = radius * Math.Sin(r);
+
+            var rectX = x + finalSize.Width / 2 - centerX;
+            var rectY = y + finalSize.Height / 2 - centerY;
+
+            element.Arrange(new Rect(rectX, rectY, element.DesiredSize.Width, element.DesiredSize.Height));
+        }
+
+        return finalSize;
+    }
+}