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/Storage/Workspace/PresetColumnCultureTests.cs

@@ -0,0 +1,59 @@
+using System.Globalization;
+using YKanBan.Storage.Workspace;
+using YKanBan.Tests.TestUtilities;
+
+namespace YKanBan.Tests.Storage.Workspace;
+
+/// <summary>
+/// Verifies that preset column titles follow the UI language active at
+/// seeding time and then live as ordinary data. Mutates the global
+/// Resources.Culture, therefore marked DoNotParallelize.
+/// </summary>
+[TestClass]
+[DoNotParallelize]
+public class PresetColumnCultureTests
+{
+    [TestMethod]
+    public void PresetTitlesAreEnglishUnderEnglishCulture()
+    {
+        CultureInfo? original = Resources.Culture;
+        try
+        {
+            Resources.Culture = new CultureInfo("en");
+            using var directory = new TempDirectory();
+            WorkspaceInitializer.Initialize(directory.FullPath);
+            WorkspacePreset.SeedColumns(directory.FullPath);
+
+            using var connection = SqliteTestHelper.OpenWorkspace(directory.FullPath);
+            CollectionAssert.AreEqual(
+                new[] { "To Do", "In Progress", "Done" },
+                SqliteTestHelper.GetColumnTitles(connection));
+        }
+        finally
+        {
+            Resources.Culture = original;
+        }
+    }
+
+    [TestMethod]
+    public void PresetTitlesAreChineseUnderChineseCulture()
+    {
+        CultureInfo? original = Resources.Culture;
+        try
+        {
+            Resources.Culture = new CultureInfo("zh-Hans");
+            using var directory = new TempDirectory();
+            WorkspaceInitializer.Initialize(directory.FullPath);
+            WorkspacePreset.SeedColumns(directory.FullPath);
+
+            using var connection = SqliteTestHelper.OpenWorkspace(directory.FullPath);
+            CollectionAssert.AreEqual(
+                new[] { "待办", "进行中", "已完成" },
+                SqliteTestHelper.GetColumnTitles(connection));
+        }
+        finally
+        {
+            Resources.Culture = original;
+        }
+    }
+}

YKanBan.Tests/Storage/Workspace/WorkspaceInitializerTests.cs

@@ -0,0 +1,53 @@
+using YKanBan.Storage;
+using YKanBan.Storage.Workspace;
+using YKanBan.Tests.TestUtilities;
+
+namespace YKanBan.Tests.Storage.Workspace;
+
+/// <summary>Tests for .ykanban folder and empty database creation.</summary>
+[TestClass]
+public class WorkspaceInitializerTests
+{
+    [TestMethod]
+    public void InitializeCreatesYkanbanFolderAndDatabase()
+    {
+        using var directory = new TempDirectory();
+
+        Assert.IsFalse(WorkspaceInitializer.IsWorkspace(directory.FullPath));
+        WorkspaceInitializer.Initialize(directory.FullPath);
+
+        Assert.IsTrue(WorkspaceInitializer.IsWorkspace(directory.FullPath));
+        Assert.IsTrue(File.Exists(WorkspacePaths.Database(directory.FullPath)));
+        // The lock file only appears once a WorkspaceLock is acquired.
+        Assert.IsFalse(File.Exists(WorkspacePaths.LockFile(directory.FullPath)));
+    }
+
+    [TestMethod]
+    public void InitializeLeavesDatabaseEmpty()
+    {
+        // Initialization creates structure only; preset content is seeded separately.
+        using var directory = new TempDirectory();
+        WorkspaceInitializer.Initialize(directory.FullPath);
+
+        using var connection = SqliteTestHelper.OpenWorkspace(directory.FullPath);
+        Assert.AreEqual(0L, SqliteTestHelper.ScalarLong(connection, "SELECT COUNT(*) FROM columns;"));
+        Assert.AreEqual(0L, SqliteTestHelper.ScalarLong(connection, "SELECT COUNT(*) FROM cards;"));
+        Assert.AreEqual(0L, SqliteTestHelper.ScalarLong(connection, "SELECT COUNT(*) FROM tags;"));
+    }
+
+    [TestMethod]
+    public void InitializeThrowsWhenDirectoryIsMissing()
+    {
+        string missing = Path.Combine(Path.GetTempPath(), "ykanban-tests-" + Guid.NewGuid().ToString("N"));
+        Assert.ThrowsExactly<WorkspaceDirectoryMissingException>(() => WorkspaceInitializer.Initialize(missing));
+    }
+
+    [TestMethod]
+    public void InitializeThrowsWhenDatabaseAlreadyExists()
+    {
+        using var directory = new TempDirectory();
+        WorkspaceInitializer.Initialize(directory.FullPath);
+
+        Assert.ThrowsExactly<InvalidOperationException>(() => WorkspaceInitializer.Initialize(directory.FullPath));
+    }
+}

YKanBan.Tests/Storage/Workspace/WorkspaceLockTests.cs

@@ -0,0 +1,57 @@
+using YKanBan.Storage;
+using YKanBan.Storage.Workspace;
+using YKanBan.Tests.TestUtilities;
+
+namespace YKanBan.Tests.Storage.Workspace;
+
+/// <summary>Tests for exclusive lock acquisition, diagnostics and release.</summary>
+[TestClass]
+public class WorkspaceLockTests
+{
+    [TestMethod]
+    public void AcquireThenDisposeThenAcquireAgainSucceeds()
+    {
+        using var directory = new TempDirectory();
+        WorkspaceInitializer.Initialize(directory.FullPath);
+
+        WorkspaceLock.Acquire(directory.FullPath).Dispose();
+        // Re-acquiring after release works: no stale lock residue survives.
+        WorkspaceLock.Acquire(directory.FullPath).Dispose();
+    }
+
+    [TestMethod]
+    public void SecondAcquireWhileHeldThrows()
+    {
+        using var directory = new TempDirectory();
+        WorkspaceInitializer.Initialize(directory.FullPath);
+
+        using WorkspaceLock first = WorkspaceLock.Acquire(directory.FullPath);
+        WorkspaceLockException ex = Assert.ThrowsExactly<WorkspaceLockException>(
+            () => WorkspaceLock.Acquire(directory.FullPath));
+        Assert.AreEqual(WorkspacePaths.LockFile(directory.FullPath), ex.LockFilePath);
+    }
+
+    [TestMethod]
+    public void AcquireWithoutYkanbanThrowsNotInitialized()
+    {
+        using var directory = new TempDirectory();
+        Assert.ThrowsExactly<WorkspaceNotInitializedException>(
+            () => WorkspaceLock.Acquire(directory.FullPath));
+    }
+
+    [TestMethod]
+    public void LockFileContainsHolderDiagnostics()
+    {
+        using var directory = new TempDirectory();
+        WorkspaceInitializer.Initialize(directory.FullPath);
+
+        using (WorkspaceLock.Acquire(directory.FullPath))
+        {
+        }
+
+        // The file survives release and carries PID / machine name / time for humans.
+        string content = File.ReadAllText(WorkspacePaths.LockFile(directory.FullPath));
+        StringAssert.Contains(content, "pid=");
+        StringAssert.Contains(content, $"machine={Environment.MachineName}");
+    }
+}

YKanBan.Tests/Storage/Workspace/WorkspacePresetTests.cs

@@ -0,0 +1,39 @@
+using YKanBan.Storage.Workspace;
+using YKanBan.Tests.TestUtilities;
+
+namespace YKanBan.Tests.Storage.Workspace;
+
+/// <summary>Tests for seeding preset workspace content.</summary>
+[TestClass]
+public class WorkspacePresetTests
+{
+    [TestMethod]
+    public void SeedAddsThreeDistinctPresetColumns()
+    {
+        using var directory = new TempDirectory();
+        WorkspaceInitializer.Initialize(directory.FullPath);
+        WorkspacePreset.SeedColumns(directory.FullPath);
+
+        using var connection = SqliteTestHelper.OpenWorkspace(directory.FullPath);
+        List<string> titles = SqliteTestHelper.GetColumnTitles(connection);
+
+        Assert.AreEqual(3, titles.Count);
+        Assert.AreEqual(titles.Count, titles.Distinct().Count());
+        Assert.IsTrue(titles.All(title => title.Length > 0));
+    }
+
+    [TestMethod]
+    public void SeedPresetColumnsShareOneTimestamp()
+    {
+        using var directory = new TempDirectory();
+        WorkspaceInitializer.Initialize(directory.FullPath);
+        WorkspacePreset.SeedColumns(directory.FullPath);
+
+        using var connection = SqliteTestHelper.OpenWorkspace(directory.FullPath);
+
+        // Creation writes created_at = updated_at; all three columns share one moment.
+        Assert.AreEqual(0L, SqliteTestHelper.ScalarLong(connection, "SELECT COUNT(*) FROM columns WHERE created_at <> updated_at;"));
+        Assert.AreEqual(3L, SqliteTestHelper.ScalarLong(connection, "SELECT COUNT(*) FROM columns WHERE created_at > 0;"));
+        Assert.AreEqual(1L, SqliteTestHelper.ScalarLong(connection, "SELECT COUNT(DISTINCT created_at) FROM columns;"));
+    }
+}

YKanBan.Tests/Storage/Workspace/WorkspaceSchemaTests.cs

@@ -0,0 +1,160 @@
+using Microsoft.Data.Sqlite;
+using YKanBan.Storage;
+using YKanBan.Storage.Workspace;
+using YKanBan.Tests.TestUtilities;
+
+namespace YKanBan.Tests.Storage.Workspace;
+
+/// <summary>Verifies that the DDL constraints reject invalid data at the door.</summary>
+[TestClass]
+public class WorkspaceSchemaTests
+{
+    /// <summary>Owns a temp directory plus a fresh workspace-schema database for one test.</summary>
+    private sealed class FreshDatabase : IDisposable
+    {
+        public TempDirectory Directory { get; } = new();
+
+        public SqliteConnection Connection { get; }
+
+        public FreshDatabase()
+        {
+            Connection = SqliteDatabase.Open(
+                Path.Combine(Directory.FullPath, "ws.db"), WorkspaceSchema.Migrations);
+        }
+
+        public void Dispose()
+        {
+            Connection.Dispose();
+            Directory.Dispose();
+        }
+    }
+
+    [TestMethod]
+    public void EmptyColumnTitleIsRejected()
+    {
+        using var db = new FreshDatabase();
+        using var connection = db.Connection;
+        Assert.ThrowsExactly<SqliteException>(() =>
+            SqliteTestHelper.Exec(connection, "INSERT INTO columns (title, created_at, updated_at) VALUES ('', 1, 1);"));
+    }
+
+    [TestMethod]
+    public void DuplicateColumnTitleIsRejected()
+    {
+        using var db = new FreshDatabase();
+        using var connection = db.Connection;
+        SqliteTestHelper.Exec(connection, "INSERT INTO columns (title, created_at, updated_at) VALUES ('Foo', 1, 1);");
+        Assert.ThrowsExactly<SqliteException>(() =>
+            SqliteTestHelper.Exec(connection, "INSERT INTO columns (title, created_at, updated_at) VALUES ('Foo', 1, 1);"));
+    }
+
+    [TestMethod]
+    public void ColumnTitlesCompareCaseSensitively()
+    {
+        using var db = new FreshDatabase();
+        using var connection = db.Connection;
+        // BINARY collation: 'Foo' and 'foo' are distinct titles.
+        SqliteTestHelper.Exec(connection, "INSERT INTO columns (title, created_at, updated_at) VALUES ('Foo', 1, 1);");
+        SqliteTestHelper.Exec(connection, "INSERT INTO columns (title, created_at, updated_at) VALUES ('foo', 1, 1);");
+        Assert.AreEqual(2L, SqliteTestHelper.ScalarLong(connection, "SELECT COUNT(*) FROM columns;"));
+    }
+
+    [TestMethod]
+    public void EmptyTagNameIsRejected()
+    {
+        using var db = new FreshDatabase();
+        using var connection = db.Connection;
+        Assert.ThrowsExactly<SqliteException>(() =>
+            SqliteTestHelper.Exec(connection, "INSERT INTO tags (name, color) VALUES ('', '#112233');"));
+    }
+
+    [TestMethod]
+    public void TagNamesAcceptSpacesEmojiAndReservedWords()
+    {
+        using var db = new FreshDatabase();
+        using var connection = db.Connection;
+        // Tag names are fully open: no charset or length rules, no trimming.
+        foreach (string name in new[] { " ", "a b", "🏷️", "AND", "tag:with:colons" })
+        {
+            SqliteTestHelper.Exec(connection, "INSERT INTO tags (name, color) VALUES ($n, '#112233');", ("$n", name));
+        }
+        Assert.AreEqual(5L, SqliteTestHelper.ScalarLong(connection, "SELECT COUNT(*) FROM tags;"));
+    }
+
+    [TestMethod]
+    public void DuplicateTagNameIsRejected()
+    {
+        using var db = new FreshDatabase();
+        using var connection = db.Connection;
+        SqliteTestHelper.Exec(connection, "INSERT INTO tags (name, color) VALUES ('bug', '#112233');");
+        Assert.ThrowsExactly<SqliteException>(() =>
+            SqliteTestHelper.Exec(connection, "INSERT INTO tags (name, color) VALUES ('bug', '#445566');"));
+    }
+
+    [TestMethod]
+    [DataRow("#000000")]
+    [DataRow("#FFFFFF")]
+    [DataRow("#a1B2c3")]
+    public void ValidTagColorsAreAccepted(string color)
+    {
+        using var db = new FreshDatabase();
+        using var connection = db.Connection;
+        int affected = SqliteTestHelper.Exec(connection, "INSERT INTO tags (name, color) VALUES ('x', $c);", ("$c", color));
+        Assert.AreEqual(1, affected);
+    }
+
+    [TestMethod]
+    [DataRow("112233")]
+    [DataRow("#11223")]
+    [DataRow("#1122334")]
+    [DataRow("#G1B2C3")]
+    [DataRow("#a1b2g3")]
+    [DataRow(" #112233")]
+    [DataRow("#112233 ")]
+    [DataRow("")]
+    public void InvalidTagColorsAreRejected(string color)
+    {
+        using var db = new FreshDatabase();
+        using var connection = db.Connection;
+        Assert.ThrowsExactly<SqliteException>(() =>
+            SqliteTestHelper.Exec(connection, "INSERT INTO tags (name, color) VALUES ('x', $c);", ("$c", color)));
+    }
+
+    [TestMethod]
+    public void CardRequiresExistingColumn()
+    {
+        using var db = new FreshDatabase();
+        using var connection = db.Connection;
+        // foreign_keys=ON makes the FK enforceable; this is the DDL contract behind the cascades.
+        Assert.ThrowsExactly<SqliteException>(() =>
+            SqliteTestHelper.Exec(connection, "INSERT INTO cards (column_id, created_at, updated_at) VALUES (999, 1, 1);"));
+    }
+
+    [TestMethod]
+    public void CardDefaultsToEmptyTitleAndContent()
+    {
+        using var db = new FreshDatabase();
+        using var connection = db.Connection;
+        SqliteTestHelper.Exec(connection, "INSERT INTO columns (title, created_at, updated_at) VALUES ('c', 1, 1);");
+        SqliteTestHelper.Exec(connection, "INSERT INTO cards (column_id, created_at, updated_at) VALUES (1, 1, 1);");
+
+        // No title is represented by the empty string.
+        Assert.AreEqual(string.Empty, SqliteTestHelper.ScalarString(connection, "SELECT title FROM cards WHERE id = 1;"));
+        Assert.AreEqual(string.Empty, SqliteTestHelper.ScalarString(connection, "SELECT content FROM cards WHERE id = 1;"));
+    }
+
+    [TestMethod]
+    public void CardTagDuplicateAssignmentIsRejected()
+    {
+        using var db = new FreshDatabase();
+        using var connection = db.Connection;
+        SqliteTestHelper.Exec(connection, "INSERT INTO columns (title, created_at, updated_at) VALUES ('c', 1, 1);");
+        SqliteTestHelper.Exec(connection, "INSERT INTO cards (column_id, created_at, updated_at) VALUES (1, 1, 1);");
+        SqliteTestHelper.Exec(connection, "INSERT INTO tags (name, color) VALUES ('bug', '#112233');");
+        SqliteTestHelper.Exec(connection, "INSERT INTO card_tags (card_id, tag_id) VALUES (1, 1);");
+
+        // Composite primary key: the same (card, tag) pair cannot be inserted twice.
+        Assert.ThrowsExactly<SqliteException>(() =>
+            SqliteTestHelper.Exec(connection, "INSERT INTO card_tags (card_id, tag_id) VALUES (1, 1);"));
+    }
+}