feat: drop created-at card sort and add card display ordering

YKanBan.Tests/Models/CardOrderingTests.cs

@@ -0,0 +1,46 @@
+using YKanBan.Models;
+using YKanBan.Storage.AppData;
+
+namespace YKanBan.Tests.Models;
+
+/// <summary>
+/// Tests for the in-column card display order: each sort option plus the id
+/// ascending tie-breaker.
+/// </summary>
+[TestClass]
+public class CardOrderingTests
+{
+    private static CardModel Card(long id, string title = "", long updatedAt = 0) =>
+        new(id, 1, title, string.Empty, 0, updatedAt, []);
+
+    private static long[] Ids(IEnumerable<CardModel> cards, CardSortOption option) =>
+        CardOrdering.Sort(cards, option).Select(card => card.Id).ToArray();
+
+    [TestMethod]
+    public void IdSortsAscending()
+    {
+        CollectionAssert.AreEqual(new[] { 1L, 2L, 3L }, Ids([Card(3), Card(1), Card(2)], CardSortOption.Id));
+    }
+
+    [TestMethod]
+    public void UpdatedAtSortsMostRecentFirstThenById()
+    {
+        CardModel[] cards = [Card(1, updatedAt: 10), Card(2, updatedAt: 30), Card(3, updatedAt: 10), Card(4, updatedAt: 20)];
+        CollectionAssert.AreEqual(new[] { 2L, 4L, 1L, 3L }, Ids(cards, CardSortOption.UpdatedAt));
+    }
+
+    [TestMethod]
+    public void TitleSortsByCodePointWithEmptyFirstThenById()
+    {
+        CardModel[] cards = [Card(1, "b"), Card(2, "B"), Card(3, ""), Card(4, "a"), Card(5, "b")];
+        CollectionAssert.AreEqual(new[] { 3L, 2L, 4L, 1L, 5L }, Ids(cards, CardSortOption.Title));
+    }
+
+    [TestMethod]
+    public void TitleSortsSupplementaryCharactersAfterBmpPrivateUse()
+    {
+        // U+1F600 is a surrogate pair in UTF-16, so ordinal comparison would put it before U+E000.
+        CardModel[] cards = [Card(1, "\U0001F600"), Card(2, "")];
+        CollectionAssert.AreEqual(new[] { 2L, 1L }, Ids(cards, CardSortOption.Title));
+    }
+}