feat(m2): repository layer

- Data models: ColumnModel, CardModel (tags loaded), TagModel
- PathComparer: platform-parameterized path normalization
  (case rule injected, trailing separator trimmed, roots kept)
- WorkspaceRepository: column/card/tag CRUD, card moves, tag
  assignment; single-transaction edit confirmation with atomic
  rollback; no-change confirmations write nothing; move refreshes
  last_edited_at but not card updated_at; write-through to app.db
  after every successful data write
- WorkspaceConnectionPool: fixed-capacity LRU session pool
  (lock + repository); eviction releases resources only; typed
  folder state errors; sidebar registry registration on open
- 35 new unit tests: cascade chains, atomicity, timestamp rules,
  LRU behavior, lock conflicts, path equivalence

YKanBan.Tests/Platform/PathComparerTests.cs

@@ -0,0 +1,93 @@
+using YKanBan.Platform;
+using YKanBan.Tests.TestUtilities;
+
+namespace YKanBan.Tests.Platform;
+
+/// <summary>Tests for path normalization and platform-parameterized comparison.</summary>
+[TestClass]
+public class PathComparerTests
+{
+    [TestMethod]
+    public void OrdinalComparerTreatsCaseAsSignificant()
+    {
+        // Unix-style rule, injected so it is testable on any host.
+        var comparer = new PathComparer(ignoreCase: false);
+        Assert.IsFalse(comparer.Equals(@"C:\A", @"C:\a"));
+    }
+
+    [TestMethod]
+    public void IgnoreCaseComparerTreatsCaseAsInsignificant()
+    {
+        // Windows-style rule.
+        var comparer = new PathComparer(ignoreCase: true);
+        Assert.IsTrue(comparer.Equals(@"C:\A", @"c:\a"));
+    }
+
+    [TestMethod]
+    public void EqualPathsProduceEqualHashCodes()
+    {
+        var comparer = new PathComparer(ignoreCase: true);
+        Assert.AreEqual(
+            comparer.GetHashCode(@"C:\Users\Data\MyRepo"),
+            comparer.GetHashCode(@"c:\users\data\myrepo"));
+    }
+
+    [TestMethod]
+    public void RelativeSegmentsCollapse()
+    {
+        var comparer = new PathComparer(ignoreCase: true);
+        using var directory = new TempDirectory();
+
+        string viaParent = Path.Combine(directory.FullPath, "x", "..", "a");
+        string direct = Path.Combine(directory.FullPath, "a");
+
+        Assert.IsTrue(comparer.Equals(viaParent, direct));
+    }
+
+    [TestMethod]
+    public void SeparatorStylesUnify()
+    {
+        var comparer = new PathComparer(ignoreCase: true);
+
+        Assert.IsTrue(comparer.Equals(@"C:\a/b\c", @"C:\a\b\c"));
+    }
+
+    [TestMethod]
+    public void TrailingSeparatorIsTrimmedForNonRoots()
+    {
+        var comparer = new PathComparer(ignoreCase: true);
+        using var directory = new TempDirectory();
+
+        string withTrailing = directory.FullPath + Path.DirectorySeparatorChar;
+        Assert.IsTrue(comparer.Equals(withTrailing, directory.FullPath));
+    }
+
+    [TestMethod]
+    public void NormalizeReturnsCanonicalAbsoluteForm()
+    {
+        var comparer = new PathComparer(ignoreCase: false);
+        using var directory = new TempDirectory();
+
+        string withRelativeSegment = Path.Combine(directory.FullPath, "sub", "..");
+        Assert.AreEqual(directory.FullPath, comparer.Normalize(withRelativeSegment));
+    }
+
+    [TestMethod]
+    public void RootPathKeepsItsTrailingSeparator()
+    {
+        var comparer = new PathComparer(ignoreCase: true);
+
+        Assert.AreEqual(@"C:\", comparer.Normalize(@"C:\"));
+        Assert.AreEqual(@"C:\x", comparer.Normalize(@"C:\x\"));
+    }
+
+    [TestMethod]
+    public void ForCurrentPlatformMatchesHostRule()
+    {
+        PathComparer comparer = PathComparer.ForCurrentPlatform();
+
+        Assert.AreEqual(
+            OperatingSystem.IsWindows(),
+            comparer.Equals(@"C:\Same", @"c:\same"));
+    }
+}