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.Audio.cs

@@ -68,9 +68,9 @@ public partial class MainViewModel
 
     public string MeterBrush => MeterLevel switch
     {
-        < 0.6 => "#38bdf8",
-        < 0.8 => "#ffb95f",
-        _ => "#ff5451",
+        < 0.6 => ThemeBrush.Hex("Brush.Meter.Nominal"),
+        < 0.8 => ThemeBrush.Hex("Brush.Meter.Caution"),
+        _ => ThemeBrush.Hex("Brush.Meter.Peak"),
     };
 
     /// <summary>Previewes the volume position while the slider is dragged; the
@@ -201,7 +201,7 @@ public partial class MainViewModel
 
     // ─── Mic status dot (the MIC button) ───
 
-    private static readonly SolidColorBrush MicProblemBrush = CreateBrush("#f1c40f");
+    private static SolidColorBrush MicProblemBrush => ThemeBrush.Get("Brush.Secondary");
 
     /// <summary>Mic connection state, driven by the mixer's MicConnected/MicFailed
     /// events and the startup device check (see StartMicCaptureAsync).</summary>
@@ -283,9 +283,9 @@ public partial class MainViewModel
 
     public string GameMeterBrush => GameMeterLevel switch
     {
-        < 0.6 => "#38bdf8",
-        < 0.8 => "#ffb95f",
-        _ => "#ff5451",
+        < 0.6 => ThemeBrush.Hex("Brush.Meter.Nominal"),
+        < 0.8 => ThemeBrush.Hex("Brush.Meter.Caution"),
+        _ => ThemeBrush.Hex("Brush.Meter.Peak"),
     };
 
     public void SetGameVolumeAdjusting(bool adjusting)