Fix UseHighContrast to use UISettings.ColorValuesChanged instead of broken API

AccessibilitySettings.HighContrastChanged always throws COMException
0x80070490 (ERROR_NOT_FOUND) in WinUI 3 desktop apps because it requires a
CoreWindow that doesn't exist in the HWND-based window model.

Replace with UISettings.ColorValuesChanged which fires reliably for
high-contrast toggles and palette changes. This is the same approach
ReactorHost.OnColorValuesChanged already uses for charting state.

Also:
- Remove redundant HighContrastChanged subscription from ReactorHost
  (OnColorValuesChanged already re-reads high contrast state)
- Delete RenderContextUserCallbackEtwBridgeTests (test removed swallowing behavior)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: codemonkeychris

src/Reactor/Core/RenderContext.cs

@@ -800,30 +800,39 @@ private HighContrastState UseHighContrastState()
     {
         var (state, _) = UseState(new HighContrastState());
 
-        // Subscribe once to HighContrastChanged and re-render when it fires.
+        // AccessibilitySettings.HighContrastChanged throws ERROR_NOT_FOUND
+        // (0x80070490) in WinUI 3 desktop apps because it requires a CoreWindow.
+        // Instead, use UISettings.ColorValuesChanged which fires reliably for
+        // system theme changes including high contrast toggles.
         UseEffect(() =>
         {
-            state.Settings ??= new global::Windows.UI.ViewManagement.AccessibilitySettings();
+            state.A11ySettings ??= new global::Windows.UI.ViewManagement.AccessibilitySettings();
+            state.UiSettings ??= new global::Windows.UI.ViewManagement.UISettings();
             var rerender = _requestRerender;
-            void OnChanged(global::Windows.UI.ViewManagement.AccessibilitySettings sender, object args)
+
+            void OnColorValuesChanged(global::Windows.UI.ViewManagement.UISettings sender, object args)
             {
-                state.IsHighContrast = sender.HighContrast;
-                state.HighContrastScheme = sender.HighContrast ? sender.HighContrastScheme : null;
+                var a11y = state.A11ySettings;
+                state.IsHighContrast = a11y.HighContrast;
+                state.HighContrastScheme = a11y.HighContrast ? a11y.HighContrastScheme : null;
                 rerender?.Invoke();
             }
-            state.Settings.HighContrastChanged += OnChanged;
+
+            state.UiSettings.ColorValuesChanged += OnColorValuesChanged;
+
             // Sync initial value
-            state.IsHighContrast = state.Settings.HighContrast;
-            state.HighContrastScheme = state.Settings.HighContrast ? state.Settings.HighContrastScheme : null;
-            return () => state.Settings.HighContrastChanged -= OnChanged;
+            state.IsHighContrast = state.A11ySettings.HighContrast;
+            state.HighContrastScheme = state.A11ySettings.HighContrast ? state.A11ySettings.HighContrastScheme : null;
+            return () => state.UiSettings.ColorValuesChanged -= OnColorValuesChanged;
         });
 
         return state;
     }
 
     private sealed class HighContrastState
     {
-        public global::Windows.UI.ViewManagement.AccessibilitySettings? Settings;
+        public global::Windows.UI.ViewManagement.AccessibilitySettings? A11ySettings;
+        public global::Windows.UI.ViewManagement.UISettings? UiSettings;
         public bool IsHighContrast;
         public string? HighContrastScheme;
     }

src/Reactor/Hosting/ReactorHost.cs

@@ -407,14 +407,15 @@ private void InitChartingState()
             _isForcedColors = _accessibilitySettings.HighContrast;
             if (_isForcedColors)
                 _forcedColorsTheme = Charting.ForcedColorsTheme.FromSystem();
-            _accessibilitySettings.HighContrastChanged += OnHighContrastChanged;
         }
         catch { /* headless / unit-test host — no accessibility settings */ }
 
         try
         {
             _uiSettings = new global::Windows.UI.ViewManagement.UISettings();
             _isReducedMotion = !_uiSettings.AnimationsEnabled;
+            // ColorValuesChanged fires for high-contrast toggles, palette
+            // changes, and AnimationsEnabled changes — covers all charting signals.
             _uiSettings.ColorValuesChanged += OnColorValuesChanged;
         }
         catch { /* headless / unit-test host — no UI settings */ }
@@ -859,15 +860,6 @@ private void OnActualThemeChanged(FrameworkElement sender, object args)
         RequestRender();
     }
 
-    private void OnHighContrastChanged(
-        global::Windows.UI.ViewManagement.AccessibilitySettings sender, object args)
-    {
-        _isForcedColors = sender.HighContrast;
-        _forcedColorsTheme = _isForcedColors ? Charting.ForcedColorsTheme.FromSystem() : null;
-        _logger?.LogDebug("High-contrast changed to {IsHighContrast} — re-rendering", _isForcedColors);
-        RequestRender();
-    }
-
     private void OnColorValuesChanged(
         global::Windows.UI.ViewManagement.UISettings sender, object args)
     {
@@ -929,8 +921,6 @@ public void Dispose()
         });
 
         // Accessibility listener cleanup
-        if (_accessibilitySettings is not null)
-            _accessibilitySettings.HighContrastChanged -= OnHighContrastChanged;
         if (_uiSettings is not null)
             _uiSettings.ColorValuesChanged -= OnColorValuesChanged;