feat(01-05): add Serilog integration tests and App.xaml.cs LogPanelSink comment

- LoggingIntegrationTests: verifies Serilog writes rolling log file with correct content
- LogPanelSink structural smoke test: confirms type implements ILogEventSink
- App.xaml.cs: added comment for LogPanelSink DI registration deferred to plan 01-06

SharepointToolbox/App.xaml.cs

@@ -36,5 +36,7 @@ public partial class App : Application
     {
         // Placeholder — services registered in subsequent plans
         services.AddSingleton<MainWindow>();
+        // LogPanelSink registered in plan 01-06 after MainWindow is created
+        // (requires RichTextBox reference from MainWindow)
     }
 }