fix(selftest): stabilize HostControlMountFunc and Tab/RadioButtons selection fixtures (#139)

Three flake clusters in `Reactor.AppTests.Host --self-test` (18/100 run-fail
rate before; 0/100 after for the targeted clusters across a 100x sweep):

- `HostCtrlFunc_Mounted/Initial/Updated`: standalone ReactorHostControl doesn't
  register with ReactorApp.ActiveHost, so Harness.Render() can't wait on its
  render loop. Use Harness.Render(200) to give wall-clock time, matching the
  sibling HostControlMountComponent/HostControlFactory fixtures.

- `TabViewSel_*` and `RadioButtonsSel_*`: TabView and RadioButtons can raise
  SelectionChanged via the dispatcher rather than synchronously in the
  SelectedIndex setter (unlike ComboBox/ListView/GridView/FlipView/Pivot).
  Pump once with Harness.Render() between the mutation and the assertion so
  the queued event lands before the counter is read.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: codemonkeychris

tests/Reactor.AppTests.Host/SelfTest/Fixtures/HostingCoverageFixtures.cs

@@ -92,9 +92,12 @@ public override async Task RunAsync()
                 );
             });
 
+            // Standalone ReactorHostControl doesn't register with ReactorApp.ActiveHost,
+            // so Harness.Render() can't wait on its render loop directly — give it
+            // wall-clock time, same as the sibling Component / Factory fixtures.
             var container = new Border { Child = hostControl };
             H.SetContent(container);
-            await Harness.Render();
+            await Harness.Render(200);
 
             var text = FindInContainer<TextBlock>(hostControl, tb => tb.Text?.StartsWith("Func:") == true);
             H.Check("HostCtrlFunc_Mounted", text is not null);
@@ -107,7 +110,7 @@ public override async Task RunAsync()
                 ((Microsoft.UI.Xaml.Automation.Provider.IInvokeProvider)
                     peer.GetPattern(Microsoft.UI.Xaml.Automation.Peers.PatternInterface.Invoke)).Invoke();
             }
-            await Harness.Render();
+            await Harness.Render(200);
             text = FindInContainer<TextBlock>(hostControl, tb => tb.Text?.StartsWith("Func:") == true);
             H.Check("HostCtrlFunc_Updated", text?.Text == "Func:world");
 

tests/Reactor.AppTests.Host/SelfTest/Fixtures/SelectionEventFixtures.cs

@@ -71,6 +71,11 @@ public override async Task RunAsync()
             count = 0; lastIndex = -1;
             if (rbs is not null) rbs.SelectedIndex = 1;
 
+            // RadioButtons can raise SelectionChanged via the dispatcher (unlike
+            // ComboBox/ListView/etc. which fire it synchronously in the setter),
+            // so pump once before asserting on the counter.
+            await Harness.Render();
+
             H.Check("RadioButtonsSel_CallbackFired", count >= 1);
             H.Check("RadioButtonsSel_PayloadIndex", lastIndex == 1);
         }
@@ -199,6 +204,11 @@ public override async Task RunAsync()
             count = 0; lastIndex = -1;
             if (tv is not null) tv.SelectedIndex = 2;
 
+            // TabView can raise SelectionChanged via the dispatcher (unlike
+            // ComboBox/ListView/etc. which fire it synchronously in the setter),
+            // so pump once before asserting on the counter.
+            await Harness.Render();
+
             H.Check("TabViewSel_CallbackFired", count >= 1);
             H.Check("TabViewSel_PayloadIndex", lastIndex == 2);
         }