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.

ViewModels/MainViewModel.cs

@@ -109,8 +109,8 @@ public partial class MainViewModel : ViewModelBase
         }
     }
 
-    private static readonly SolidColorBrush BarOnBrush = CreateBrush("#2ecc71");
-    private static readonly SolidColorBrush BarOffBrush = CreateBrush("#e94560");
+    private static SolidColorBrush BarOnBrush => ThemeBrush.Get("Brush.Positive");
+    private static SolidColorBrush BarOffBrush => ThemeBrush.Get("Brush.Primary");
     private static SolidColorBrush CreateBrush(string hex)
         => (SolidColorBrush)new BrushConverter().ConvertFromString(hex)!;