fix(ui): D.2b inventory finish — grid cells exempt from the anchor pass

Root cause of the "grid escapes the window when scrolled" bug: UiElement.
DrawSelfAndChildren runs ApplyAnchor on every child AFTER OnDraw, which captured
each cell's scroll-0 position and reset Top to it every frame, fighting
LayoutCells' scroll offset. Cells are laid out procedurally by the list, so they
must be exempt — AddItem now sets cell.Anchors = None, making LayoutCells the
sole authority. Regression test reproduces the anchor-pass interaction (the unit
tests missed it by calling LayoutCells in isolation, never the draw traversal).

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

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

@@ -52,6 +52,24 @@ public sealed class UiItemListScrollTests
         Assert.True(list.GetItem(18)!.Visible);         // row 3 now visible
     }
 
+    [Fact]
+    public void Scroll_survives_the_per_frame_anchor_pass()
+    {
+        // Regression (the escaping-grid bug): UiElement.DrawSelfAndChildren runs ApplyAnchor
+        // on every child AFTER OnDraw/LayoutCells. Grid cells must be exempt (Anchors=None) so
+        // the anchor pass can't reset the scroll offset LayoutCells applied — otherwise the
+        // grid translates out of the view when scrolled.
+        var list = Grid(30);                                  // cells added (Anchors=None) + laid out
+        for (int i = 0; i < list.GetNumUIItems(); i++)        // first anchor pass (would capture base)
+            list.GetItem(i)!.ApplyAnchor(list.Width, list.Height);
+        list.Scroll.SetScrollY(32);                           // scroll one row
+        list.LayoutCells();
+        for (int i = 0; i < list.GetNumUIItems(); i++)        // second anchor pass (the draw traversal)
+            list.GetItem(i)!.ApplyAnchor(list.Width, list.Height);
+        Assert.Equal(0f, list.GetItem(6)!.Top);               // row 1 stays at the view top
+        Assert.False(list.GetItem(0)!.Visible);               // row 0 stays clipped above
+    }
+
     [Fact]
     public void Wheel_down_scrolls_toward_the_bottom()
     {