design: the last VM-side colours come from tokens (ThemeBrush)

The state colours the view-models hand to bindings — top bar, REC/ON-AIR dots, health banner,
meter bands, toast tints — were hex literals in C#. They now resolve from Themes/Tokens.xaml
through `Helpers/ThemeBrush`, so the values live in one place. Without an Application (unit tests)
the helper returns a neutral grey: visibly wrong rather than silently plausible.

Four tokens the code needed and the document lacked, added to DESIGN.md (so the doc-is-the-contract
guard demanded their brushes): info-container #3a3b52, success-container #1d5c38,
warning-container #b8860b, test-container #9c6f1c (the test-stream gold bar).

Also fixed by this: the top bar still painted the RETIRED brand red #e94560 when live — the
standard's #ff5451 now, and `OnAirSignTests` (which pinned the old value) says so. BarOn/BarOff
became properties rather than static fields, because a static field would resolve tokens before
the Application exists.

Verified: DesignTokensTests, GameMeterHonestyTests, NotificationAreaIntegrationTests,
OnAirSignTests, SessionTeardownTests — 38 tests green.

Services/NotificationService.cs

@@ -20,10 +20,10 @@ public sealed class NotificationService : INotificationService
     internal static readonly IReadOnlyDictionary<AppNotificationSeverity, (string BackgroundHex, double ExpirationSeconds)> Cards
         = new Dictionary<AppNotificationSeverity, (string, double)>
         {
-            [AppNotificationSeverity.Info] = ("#3a3b52", 4),
-            [AppNotificationSeverity.Success] = ("#1d5c38", 4),
-            [AppNotificationSeverity.Warning] = ("#b8860b", 8),
-            [AppNotificationSeverity.Error] = ("#8f1f1f", 0),
+            [AppNotificationSeverity.Info] = (ThemeBrush.Hex("Brush.InfoContainer"), 4),
+            [AppNotificationSeverity.Success] = (ThemeBrush.Hex("Brush.SuccessContainer"), 4),
+            [AppNotificationSeverity.Warning] = (ThemeBrush.Hex("Brush.WarningContainer"), 8),
+            [AppNotificationSeverity.Error] = (ThemeBrush.Hex("Brush.ErrorContainer"), 0),
         };
 
     private readonly NotificationManager _manager = new();
@@ -54,7 +54,7 @@ public sealed class NotificationService : INotificationService
             .Create(title, message)
             .InArea(AreaName)
             .WithBackground(NotificationColor.FromHex(backgroundHex))
-            .WithForeground(NotificationColor.FromHex("#FFFFFF"));
+            .WithForeground(NotificationColor.FromHex(ThemeBrush.Hex("Brush.OnSurface")));
 
         if (severity == AppNotificationSeverity.Error)
             builder.NeverExpires();