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>

src/AcDream.App/UI/UiItemList.cs

@@ -49,9 +49,48 @@ public sealed class UiItemList : UiElement
     public void AddItem(UiItemSlot cell)
     {
         cell.SpriteResolve ??= SpriteResolve;
-        cell.Left = 0; cell.Top = 0; cell.Width = Width; cell.Height = Height;
         _cells.Add(cell);
         AddChild(cell);
+        LayoutCells();
+    }
+
+    /// <summary>Grid columns (row-major). 1 = single column. Ignored in fill mode.</summary>
+    public int Columns { get; set; } = 1;
+
+    /// <summary>Fixed cell width in grid mode. 0 = "fill the list" — the single cell sizes
+    /// to the whole list (the toolbar single-slot legacy). Set &gt;0 (with CellHeight) for a grid.</summary>
+    public float CellWidth { get; set; }
+    /// <summary>Fixed cell height in grid mode (pairs with CellWidth).</summary>
+    public float CellHeight { get; set; }
+
+    /// <summary>Row-major pixel offset of cell <paramref name="index"/> in a grid of
+    /// <paramref name="columns"/> columns at the given cell pitch.</summary>
+    internal static (float x, float y) CellOffset(int index, int columns, float cellW, float cellH)
+    {
+        int col = index % columns, row = index / columns;
+        return (col * cellW, row * cellH);
+    }
+
+    /// <summary>Position every cell per the current mode: fill (CellWidth&lt;=0) sizes the single
+    /// cell to the list; grid (CellWidth&gt;0) tiles cells row-major at the cell pitch.</summary>
+    private void LayoutCells()
+    {
+        if (CellWidth <= 0f)
+        {
+            if (_cells.Count > 0)
+            {
+                var c = _cells[0];
+                c.Left = 0; c.Top = 0; c.Width = Width; c.Height = Height;
+            }
+            return;
+        }
+        int cols = Columns < 1 ? 1 : Columns;
+        for (int i = 0; i < _cells.Count; i++)
+        {
+            var (x, y) = CellOffset(i, cols, CellWidth, CellHeight);
+            var cell = _cells[i];
+            cell.Left = x; cell.Top = y; cell.Width = CellWidth; cell.Height = CellHeight;
+        }
     }
 
     public void Flush()
@@ -62,14 +101,8 @@ public sealed class UiItemList : UiElement
 
     protected override void OnDraw(UiRenderContext ctx)
     {
-        // The factory sets THIS list's Width/Height AFTER construction, so the cell
-        // (added in the ctor) starts 0x0. For the single-cell toolbar slot, keep the
-        // cell sized to the list each frame; the cell paints itself in the children
-        // pass that follows. (N-cell grid layout is the inventory phase.)
-        if (_cells.Count > 0)
-        {
-            var cell = _cells[0];
-            cell.Left = 0; cell.Top = 0; cell.Width = Width; cell.Height = Height;
-        }
+        // The factory sets Width/Height AFTER construction, so re-layout each frame:
+        // fill mode keeps the single toolbar cell sized to the list; grid mode tiles cells.
+        LayoutCells();
     }
 }

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);
+    }
+}