fix(D.2b): windows not anchor-managed (regression: move/resize was reset each frame)

The anchor pass added in f911b5f runs on every element's children — including
UiRoot's children, which are the top-level WINDOWS. With the default Left|Top
anchor, ApplyAnchor reset each window's Left/Top/Width/Height back to its
captured design rect EVERY frame, so user move/resize was undone instantly ("I
can't resize or move it"). A window is user-positioned, so it must not be
anchor-managed by its parent: set UiNineSlicePanel.Anchors = None. Children
INSIDE the window still anchor to it (the bars keep stretching with width).

Regression tests: UiNineSlicePanel.Anchors == None; ApplyAnchor(None) is a no-op.

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

src/AcDream.App/UI/UiNineSlicePanel.cs

@@ -29,6 +29,11 @@ public sealed class UiNineSlicePanel : UiPanel
         BorderColor = Vector4.Zero;
         Draggable = true; // retail windows are movable
         Resizable = true; // retail windows are resizable
+        // A top-level window is USER-positioned: it must NOT be anchor-managed
+        // by its parent (UiRoot), or the per-frame anchor pass would reset its
+        // Left/Top/Width/Height every frame and undo move/resize. Children
+        // INSIDE the window still anchor to it (the bars stretch with width).
+        Anchors = AnchorEdges.None;
     }
 
     /// <summary>

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

@@ -5,6 +5,26 @@ namespace AcDream.App.Tests.UI;
 
 public class UiRootInputTests
 {
+    [Fact]
+    public void UiNineSlicePanel_IsNotAnchorManaged_SoUserMoveResizeSticks()
+    {
+        // Regression: the per-frame anchor pass must NOT reset a window's rect,
+        // or move/resize get undone every frame. Windows are user-positioned.
+        var panel = new UiNineSlicePanel(_ => ((uint)1, 32, 32));
+        Assert.Equal(AnchorEdges.None, panel.Anchors);
+    }
+
+    [Fact]
+    public void ApplyAnchor_None_IsNoOp()
+    {
+        var e = new UiPanel { Left = 50, Top = 60, Width = 100, Height = 40, Anchors = AnchorEdges.None };
+        e.ApplyAnchor(800, 600);
+        Assert.Equal(50f, e.Left);
+        Assert.Equal(60f, e.Top);
+        Assert.Equal(100f, e.Width);
+        Assert.Equal(40f, e.Height);
+    }
+
     [Fact]
     public void WantsMouse_TrueOverWidget_FalseOverEmptySpace()
     {