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>

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

@@ -0,0 +1,63 @@
+using AcDream.App.UI;
+using Xunit;
+
+namespace AcDream.App.Tests.UI;
+
+public class UiCollapsibleFrameTests
+{
+    private static UiCollapsibleFrame MakeFrame(out UiPanel row2a, out UiPanel row2b)
+    {
+        var f = new UiCollapsibleFrame(_ => (1u, 1, 1))
+        {
+            CollapsedHeight = 96f,
+            ExpandedHeight  = 128f,
+        };
+        row2a = new UiPanel(); row2b = new UiPanel();
+        f.SecondRow = new UiElement[] { row2a, row2b };
+        return f;
+    }
+
+    [Fact]
+    public void Tick_belowMidpoint_snapsCollapsed_hidesSecondRow()
+    {
+        var f = MakeFrame(out var a, out var b);
+        f.Height = 100f;                 // nearer the collapsed stop (midpoint 112)
+        f.TickForTest(0.016);
+        Assert.Equal(96f, f.Height);
+        Assert.False(a.Visible);
+        Assert.False(b.Visible);
+        Assert.False(f.IsExpanded);
+    }
+
+    [Fact]
+    public void Tick_aboveMidpoint_snapsExpanded_showsSecondRow()
+    {
+        var f = MakeFrame(out var a, out var b);
+        f.Height = 120f;                 // nearer the expanded stop
+        f.TickForTest(0.016);
+        Assert.Equal(128f, f.Height);
+        Assert.True(a.Visible);
+        Assert.True(b.Visible);
+        Assert.True(f.IsExpanded);
+    }
+
+    [Fact]
+    public void Tick_notConfigured_isNoOp()
+    {
+        var f = new UiCollapsibleFrame(_ => (1u, 1, 1)); // Collapsed==Expanded==0
+        f.Height = 50f;
+        f.TickForTest(0.016);
+        Assert.Equal(50f, f.Height);     // unchanged, no divide/no forced height
+    }
+
+    [Fact]
+    public void HitEdges_respectsResizableEdgesMask_bottomOnly()
+    {
+        var panel = new UiPanel { Left = 100, Top = 100, Width = 200, Height = 100,
+            Resizable = true, ResizableEdges = ResizeEdges.Bottom };
+        // bottom edge (y=200) → Bottom only
+        Assert.Equal(ResizeEdges.Bottom, UiRoot.HitEdges(panel, 200, 200, 5));
+        // top edge (y=100) → masked out → None
+        Assert.Equal(ResizeEdges.None, UiRoot.HitEdges(panel, 200, 100, 5));
+    }
+}