feat(m1): storage layer

- Shared SQLite plumbing: PRAGMAs (WAL/NORMAL/foreign_keys) and
  user_version migrations with typed SchemaVersionException
- Workspace folder: .ykanban initialization, empty schema with
  DDL-level constraints, preset column seeding (ResX-driven),
  exclusive lock file with holder diagnostics
- AppData folder: app.db workspace registry (injectable clock),
  app.json config store with kebab-case JSON names via explicit
  [JsonProperty]/[EnumMember], nested sections, corrupt-file backup
- ResX infrastructure under Assets/Locales (en + zh-Hans) with
  MSBuild strongly-typed public Resources class
- 54 unit tests: DDL constraints, migrations, PRAGMAs, lock
  exclusivity, config degradation, culture-aware presets

YKanBan.Tests/TestUtilities/TempDirectory.cs

@@ -0,0 +1,31 @@
+namespace YKanBan.Tests.TestUtilities;
+
+/// <summary>
+/// Creates a unique temporary directory per test instance and removes it on
+/// disposal. Retries the deletion once because SQLite WAL files can linger
+/// for a moment after connections close.
+/// </summary>
+public sealed class TempDirectory : IDisposable
+{
+    /// <summary>Unique directory path managed by this instance.</summary>
+    public string FullPath { get; }
+
+    public TempDirectory()
+    {
+        FullPath = Path.Combine(Path.GetTempPath(), "ykanban-tests-" + Guid.NewGuid().ToString("N"));
+        Directory.CreateDirectory(FullPath);
+    }
+
+    public void Dispose()
+    {
+        try
+        {
+            Directory.Delete(FullPath, recursive: true);
+        }
+        catch (IOException)
+        {
+            Thread.Sleep(100);
+            try { Directory.Delete(FullPath, recursive: true); } catch (IOException) { }
+        }
+    }
+}