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>

src/AcDream.App/UI/UiItemList.cs

@@ -53,6 +53,12 @@ public sealed class UiItemList : UiElement
     public void AddItem(UiItemSlot cell)
     {
         cell.SpriteResolve ??= SpriteResolve;
+        // The list lays cells out procedurally (grid offset + scroll clip), so cells must be
+        // EXEMPT from the parent anchor pass — UiElement.DrawSelfAndChildren runs ApplyAnchor
+        // on every child after OnDraw, which would otherwise reset the scroll offset to each
+        // cell's captured base position (the escaping-grid bug). Anchors=None makes LayoutCells
+        // the sole authority over cell rects.
+        cell.Anchors = AnchorEdges.None;
         _cells.Add(cell);
         AddChild(cell);
         LayoutCells();