design: white ink on any accent fill (on-accent token)

The EDIT button's armed face used on-tertiary (#0b1326 navy) on blue — the creator read it as grey
on blue and could not read it. Same problem, latent, on the armed REC/ON-AIR pills, which used the
error ink.

One rule instead of two exceptions: a saturated accent fill carries WHITE ink. New token
on-accent #ffffff, used by the EDIT trigger and RecordPillToggle; on-tertiary stays for large blue
surfaces where dark ink is right. UiShellTests pins the EDIT foreground to it.

Themes/Controls.xaml

@@ -770,7 +770,7 @@
         <Style.Triggers>
             <Trigger Property="IsChecked" Value="True">
                 <Setter Property="Background" Value="{StaticResource Brush.Primary}"/>
-                <Setter Property="Foreground" Value="{StaticResource Brush.OnError}"/>
+                <Setter Property="Foreground" Value="{StaticResource Brush.OnAccent}"/>
             </Trigger>
         </Style.Triggers>
     </Style>

Themes/Tokens.xaml

@@ -34,6 +34,7 @@
     <!-- Accent roles: one per meaning, never mixed -->
     <SolidColorBrush x:Key="Brush.Tertiary" Color="#38bdf8"/>
     <SolidColorBrush x:Key="Brush.OnTertiary" Color="#0b1326"/>
+    <SolidColorBrush x:Key="Brush.OnAccent" Color="#ffffff"/>
     <SolidColorBrush x:Key="Brush.Primary" Color="#ff5451"/>
     <SolidColorBrush x:Key="Brush.PrimaryContainer" Color="#b91a24"/>
     <SolidColorBrush x:Key="Brush.Secondary" Color="#ffb95f"/>