design: the theme's 4px corners come from the radius token

Six control corners used a literal 4px while DESIGN.md defines radius-sm for exactly that role.
Component geometry stays literal on purpose (the pill toggle's 9px is half its 18px height — a
token there would be a lie about what the number means).

Themes/Controls.xaml

@@ -26,11 +26,11 @@
                 <ControlTemplate TargetType="Button">
                     <Grid>
                         <Border x:Name="Bd" Background="{TemplateBinding Background}"
-                                CornerRadius="4"
+                                CornerRadius="{StaticResource Radius.Sm}"
                                 Padding="{TemplateBinding Padding}">
                             <ContentPresenter HorizontalAlignment="Center" VerticalAlignment="Center"/>
                         </Border>
-                        <Border x:Name="Glow" Background="White" CornerRadius="4" Opacity="0"/>
+                        <Border x:Name="Glow" Background="White" CornerRadius="{StaticResource Radius.Sm}" Opacity="0"/>
                     </Grid>
                     <ControlTemplate.Triggers>
                         <Trigger Property="IsMouseOver" Value="True">
@@ -113,7 +113,7 @@
                     <Border x:Name="Bd" Background="{TemplateBinding Background}"
                             BorderBrush="{TemplateBinding BorderBrush}"
                             BorderThickness="{TemplateBinding BorderThickness}"
-                            CornerRadius="4" Padding="{TemplateBinding Padding}">
+                            CornerRadius="{StaticResource Radius.Sm}" Padding="{TemplateBinding Padding}">
                         <ScrollViewer x:Name="PART_ContentHost" Margin="0"/>
                     </Border>
                     <ControlTemplate.Triggers>
@@ -239,7 +239,7 @@
                         <Border x:Name="Chrome" Background="{TemplateBinding Background}"
                                 BorderBrush="{TemplateBinding BorderBrush}"
                                 BorderThickness="{TemplateBinding BorderThickness}"
-                                CornerRadius="4"/>
+                                CornerRadius="{StaticResource Radius.Sm}"/>
                         <ContentPresenter x:Name="ContentSite" IsHitTestVisible="False"
                                           Margin="{TemplateBinding Padding}"
                                           HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
@@ -314,7 +314,7 @@
         <Setter Property="Template">
             <Setter.Value>
                 <ControlTemplate TargetType="Thumb">
-                    <Border x:Name="ThumbBd" Background="{StaticResource Brush.SurfaceBright}" CornerRadius="4" Margin="2"/>
+                    <Border x:Name="ThumbBd" Background="{StaticResource Brush.SurfaceBright}" CornerRadius="{StaticResource Radius.Sm}" Margin="2"/>
                     <ControlTemplate.Triggers>
                         <Trigger Property="IsMouseOver" Value="True">
                             <Setter TargetName="ThumbBd" Property="Background" Value="{StaticResource Brush.SurfaceBright}"/>
@@ -475,7 +475,7 @@
                     <Border Background="{TemplateBinding Background}"
                             BorderBrush="{TemplateBinding BorderBrush}"
                             BorderThickness="{TemplateBinding BorderThickness}"
-                            CornerRadius="4" Padding="{TemplateBinding Padding}">
+                            CornerRadius="{StaticResource Radius.Sm}" Padding="{TemplateBinding Padding}">
                         <ContentPresenter/>
                     </Border>
                 </ControlTemplate>