feat(ui): D.2b — toolbar collapse-to-one-row (bottom-edge snap resize hides/shows row 2)

UiElement.MaxHeight + ResizableEdges mask; UiCollapsibleFrame snaps height to the nearer
of {collapsed,expanded} and toggles row-2 visibility; GameWindow computes the two heights
from the layout + top-anchors the content. Amends IA-17. UiNineSlicePanel unsealed to
allow subclassing.

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

src/AcDream.App/UI/UiNineSlicePanel.cs

@@ -10,7 +10,7 @@ namespace AcDream.App.UI;
 /// the widget is testable without GL. In production:
 /// <c>id => { var t = cache.GetOrUploadRenderSurface(id, out var w, out var h); return (t, w, h); }</c>.
 /// </summary>
-public sealed class UiNineSlicePanel : UiPanel
+public class UiNineSlicePanel : UiPanel
 {
     /// <summary>A placed chrome piece: destination rect in local pixel space.</summary>
     public readonly record struct Rect(float X, float Y, float W, float H);