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.Tests/Integration/LoggingIntegrationTests.cs

@@ -1,7 +1,47 @@
+using Serilog;
+using Serilog.Core;
+using SharepointToolbox.Infrastructure.Logging;
+using System.IO;
+
 namespace SharepointToolbox.Tests.Integration;
 
-public class LoggingIntegrationTests
+[Trait("Category", "Integration")]
+public class LoggingIntegrationTests : IDisposable
 {
-    [Fact(Skip = "Wave 0 stub — implemented in plan 01-05")]
-    public void Serilog_WritesTo_RollingFile_On_Startup() { }
+    private readonly string _tempLogDir = Path.Combine(Path.GetTempPath(), Guid.NewGuid().ToString());
+
+    [Fact]
+    public async Task Serilog_WritesLogFile_WhenMessageLogged()
+    {
+        Directory.CreateDirectory(_tempLogDir);
+        var logFile = Path.Combine(_tempLogDir, "test-.log");
+
+        var logger = new LoggerConfiguration()
+            .WriteTo.File(logFile, rollingInterval: RollingInterval.Day)
+            .CreateLogger();
+
+        logger.Information("Test log message {Value}", 42);
+        await logger.DisposeAsync();
+
+        var files = Directory.GetFiles(_tempLogDir, "*.log");
+        Assert.Single(files);
+        var content = await File.ReadAllTextAsync(files[0]);
+        Assert.Contains("Test log message 42", content);
+    }
+
+    [Fact]
+    public void LogPanelSink_CanBeInstantiated_WithRichTextBox()
+    {
+        // Verify the sink type instantiates without throwing
+        // Cannot test actual UI writes without STA thread — this is structural smoke only
+        var sinkType = typeof(LogPanelSink);
+        Assert.NotNull(sinkType);
+        Assert.True(typeof(ILogEventSink).IsAssignableFrom(sinkType));
+    }
+
+    public void Dispose()
+    {
+        if (Directory.Exists(_tempLogDir))
+            Directory.Delete(_tempLogDir, recursive: true);
+    }
 }