feat(ui): D.2b-B — UiItemList N-cell grid mode

Columns + CellWidth/CellHeight + a row-major CellOffset/LayoutCells pass.
CellWidth<=0 keeps the single-cell fill mode (toolbar slot sizes to the
list — unchanged); CellWidth>0 tiles cells in a grid. Layout runs on
AddItem and per-frame in OnDraw so a list resize reflows.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/AcDream.App.Tests/UI/UiItemListGridTests.cs

@@ -0,0 +1,43 @@
+using AcDream.App.UI;
+
+namespace AcDream.App.Tests.UI;
+
+public class UiItemListGridTests
+{
+    [Fact]
+    public void CellOffset_RowMajor()
+    {
+        Assert.Equal((0f, 0f),   UiItemList.CellOffset(0, 3, 36, 36));
+        Assert.Equal((72f, 0f),  UiItemList.CellOffset(2, 3, 36, 36));
+        Assert.Equal((36f, 36f), UiItemList.CellOffset(4, 3, 36, 36)); // col 1, row 1
+    }
+
+    [Fact]
+    public void GridMode_PositionsCellsInColumns()
+    {
+        var list = new UiItemList { Columns = 3, CellWidth = 36, CellHeight = 36 };
+        list.Flush();                                   // drop the ctor's default cell
+        for (int i = 0; i < 7; i++) list.AddItem(new UiItemSlot());
+
+        var c4 = list.GetItem(4)!;
+        Assert.Equal(36f, c4.Left);
+        Assert.Equal(36f, c4.Top);
+        Assert.Equal(36f, c4.Width);
+        Assert.Equal(36f, c4.Height);
+    }
+
+    [Fact]
+    public void FillMode_SizesSingleCellToList()
+    {
+        // CellWidth defaults to 0 = "fill the list" (single-cell toolbar legacy).
+        var list = new UiItemList { Width = 36, Height = 36 };
+        list.Flush();
+        list.AddItem(new UiItemSlot());
+
+        var c = list.Cell;
+        Assert.Equal(0f, c.Left);
+        Assert.Equal(0f, c.Top);
+        Assert.Equal(36f, c.Width);
+        Assert.Equal(36f, c.Height);
+    }
+}