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.

Controls/TopBar.xaml

@@ -68,7 +68,7 @@
                         <ControlTemplate.Triggers>
                             <Trigger Property="IsChecked" Value="True">
                                 <Setter TargetName="EditBody" Property="Background" Value="{StaticResource Brush.Tertiary}"/>
-                                <Setter Property="Foreground" Value="{StaticResource Brush.OnTertiary}"/>
+                                <Setter Property="Foreground" Value="{StaticResource Brush.OnAccent}"/>
                             </Trigger>
                         </ControlTemplate.Triggers>
                     </ControlTemplate>