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.

ytLive.Tests/UiShellTests.cs

@@ -30,7 +30,7 @@ public class UiShellTests
         // 2. EDIT: resting face differs from the armed face, and the label follows it.
         Assert.Contains("x:Name=\"EditBody\" Background=\"{StaticResource Brush.SurfaceContainerHigh}\"", topBar);
         Assert.Contains("<Setter TargetName=\"EditBody\" Property=\"Background\" Value=\"{StaticResource Brush.Tertiary}\"/>", topBar);
-        Assert.Contains("<Setter Property=\"Foreground\" Value=\"{StaticResource Brush.OnTertiary}\"/>", topBar);
+        Assert.Contains("<Setter Property=\"Foreground\" Value=\"{StaticResource Brush.OnAccent}\"/>", topBar);
 
         // 3. Active scene = orange; live stays red.
         Assert.Contains("IsStaged}\" Value=\"True\">\n                                        <Setter Property=\"BorderBrush\" Value=\"{StaticResource Brush.Attention}\"/>", strip);