fix: restore async WriteAsync with proper write chaining

Address PR review feedback:
- Restore fire-and-forget WriteAsync per reviewer request (jasonleenaylor)
- Chain writes via ContinueWith/Unwrap to prevent overlapping async
  writes on the same stream
- Use bytes.Length for correct byte count (fixes truncation on Windows
  where Environment.NewLine is 2 chars)
- Dispose waits for pending writes via GetAwaiter().GetResult() before
  flushing and closing the stream
- Remove unnecessary Thread.Sleep(200) from test; Dispose guarantees
  writes are complete

Author: johnml1135

src/SIL.LCModel/FileTransactionLogger.cs

@@ -2,13 +2,15 @@
 using System.Diagnostics;
 using System.IO;
 using System.Text;
+using System.Threading.Tasks;
 
 namespace SIL.LCModel
 {
    internal class FileTransactionLogger : ITransactionLogger, IDisposable
    {
 	   private readonly object m_lock = new object();
 	   private FileStream m_stream;
+	   private Task m_lastWrite = Task.CompletedTask;
 	   private bool m_disposed;
 
 	   internal FileTransactionLogger(string filePath)
@@ -30,8 +32,9 @@ public void AddBreadCrumb(string description)
 			   if (m_disposed)
 				   return;
 			   var bytes = Encoding.UTF8.GetBytes(description + Environment.NewLine);
-			   m_stream.Write(bytes, 0, bytes.Length);
-			   m_stream.Flush();
+			   m_lastWrite = m_lastWrite.ContinueWith(_ =>
+				   m_stream.WriteAsync(bytes, 0, bytes.Length),
+				   TaskContinuationOptions.ExecuteSynchronously).Unwrap();
 		   }
 	   }
 
@@ -42,6 +45,8 @@ protected virtual void Dispose(bool disposing)
 			{
 				if (!m_disposed)
 				{
+					try { m_lastWrite?.GetAwaiter().GetResult(); }
+					catch { /* best-effort: don't let a failed write prevent disposal */ }
 					m_stream?.Flush();
 					m_stream?.Dispose();
 					m_stream = null;

tests/SIL.LCModel.Tests/FileTransactionLoggerTests.cs

@@ -73,9 +73,7 @@ public void AddBreadCrumb_WritesFullMessageIncludingNewline()
 				logger.AddBreadCrumb(message);
 			}
 
-			// Wait briefly for async write, then read
-			System.Threading.Thread.Sleep(200);
-
+			// Dispose flushes the stream, so the file is readable immediately.
 			string content = File.ReadAllText(m_tempFile, Encoding.UTF8);
 			string expected = message + Environment.NewLine;
 			Assert.AreEqual(expected, content,