refactor: organize test project structure

SongTidy.Tests/TestUtilities/TempDirectory.cs

@@ -0,0 +1,40 @@
+namespace SongTidy.Tests.TestUtilities;
+
+/// <summary>
+/// Creates a unique temporary directory per test instance and removes it on disposal.
+/// </summary>
+public sealed class TempDirectory : IDisposable {
+    /// <summary>
+    /// Gets the unique directory path managed by this instance.
+    /// </summary>
+    public string FullPath { get; }
+
+    /// <summary>
+    /// Creates a unique temporary directory.
+    /// </summary>
+    public TempDirectory() {
+        FullPath = Path.Combine(Path.GetTempPath(), "songtidy-tests-" + Guid.NewGuid().ToString("N"));
+        Directory.CreateDirectory(FullPath);
+    }
+
+    /// <summary>
+    /// Deletes the directory tree, retrying once after a short delay.
+    /// </summary>
+    public void Dispose() {
+        try {
+            // SAFETY: Recursive deletion is confined to the GUID-named directory
+            // created by this instance under the OS temp folder.
+            Directory.Delete(FullPath, recursive: true);
+        } catch (IOException) {
+            // A file handle may still be closing; retry once before giving up.
+            Thread.Sleep(100);
+
+            try {
+                // SAFETY: The retry targets the same instance-owned temp directory.
+                Directory.Delete(FullPath, recursive: true);
+            } catch (IOException) {
+                // Leftovers in the OS temp folder are harmless; never fail a test over cleanup.
+            }
+        }
+    }
+}