feat(ui): D.2b inventory finish — UiItemList clip+scroll via UiScrollable

Grid mode now drives a shared UiScrollable from its content + clips whole rows
to the panel (cells offset by -ScrollY, Visible toggled by whole-row clip,
mirroring UiText). Fill mode (toolbar single cell) unchanged. LayoutCells made
internal for tests; RowCount helper added.

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

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

@@ -0,0 +1,54 @@
+using AcDream.App.UI;
+using Xunit;
+
+namespace AcDream.App.Tests.UI;
+
+public sealed class UiItemListScrollTests
+{
+    private static UiItemList Grid(int items)
+    {
+        var list = new UiItemList { Columns = 6, CellWidth = 32, CellHeight = 32, Width = 192, Height = 96 };
+        list.Flush();                                   // drop the default single cell
+        for (int i = 0; i < items; i++) list.AddItem(new UiItemSlot());
+        list.LayoutCells();                             // drive the scroll model + position cells
+        return list;
+    }
+
+    [Fact]
+    public void RowCount_ceils_to_whole_rows()
+    {
+        Assert.Equal(0, UiItemList.RowCount(0, 6));
+        Assert.Equal(1, UiItemList.RowCount(1, 6));
+        Assert.Equal(7, UiItemList.RowCount(41, 6));
+    }
+
+    [Fact]
+    public void Grid_drives_scroll_model_from_content()
+    {
+        var list = Grid(30);                            // 5 rows × 32 = 160 content, 96 view
+        Assert.Equal(160, list.Scroll.ContentHeight);
+        Assert.Equal(96, list.Scroll.ViewHeight);
+        Assert.Equal(64, list.Scroll.MaxScroll);
+        Assert.True(list.Scroll.HasOverflow);
+    }
+
+    [Fact]
+    public void At_top_first_three_rows_visible_rest_clipped()
+    {
+        var list = Grid(30);
+        Assert.True(list.GetItem(0)!.Visible);          // row 0, top 0
+        Assert.True(list.GetItem(12)!.Visible);         // row 2, top 64 (+32 == 96)
+        Assert.False(list.GetItem(18)!.Visible);        // row 3, top 96 (off the bottom)
+    }
+
+    [Fact]
+    public void Scrolled_one_row_shifts_window_and_clips_top_row()
+    {
+        var list = Grid(30);
+        list.Scroll.SetScrollY(32);
+        list.LayoutCells();
+        Assert.False(list.GetItem(0)!.Visible);         // row 0 scrolled above
+        Assert.Equal(0f, list.GetItem(6)!.Top);         // row 1 now at the view top
+        Assert.True(list.GetItem(18)!.Visible);         // row 3 now visible
+    }
+}