feat(D.2b): UiScrollable — pixel scroll model (UIElement_Scrollable port)

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

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

@@ -0,0 +1,73 @@
+using AcDream.App.UI;
+using Xunit;
+
+namespace AcDream.App.Tests.UI;
+
+public class UiScrollableTests
+{
+    [Fact]
+    public void Clamp_KeepsScrollWithinContent()
+    {
+        var s = new UiScrollable { ContentHeight = 300, ViewHeight = 100 };
+        s.SetScrollY(500);
+        Assert.Equal(200, s.ScrollY);
+        s.SetScrollY(-50);
+        Assert.Equal(0, s.ScrollY);
+    }
+
+    [Fact]
+    public void FitsView_PinsToZero()
+    {
+        var s = new UiScrollable { ContentHeight = 80, ViewHeight = 100 };
+        s.SetScrollY(40);
+        Assert.Equal(0, s.ScrollY);
+        Assert.False(s.HasOverflow);
+    }
+
+    [Fact]
+    public void ThumbRatio_IsViewOverContent_ClampedToOne()
+    {
+        var s = new UiScrollable { ContentHeight = 400, ViewHeight = 100 };
+        Assert.Equal(0.25f, s.ThumbRatio, 3);
+        var full = new UiScrollable { ContentHeight = 50, ViewHeight = 100 };
+        Assert.Equal(1f, full.ThumbRatio, 3);
+    }
+
+    [Fact]
+    public void PositionRatio_MapsScrollToZeroOne()
+    {
+        var s = new UiScrollable { ContentHeight = 300, ViewHeight = 100 };
+        s.SetScrollY(100);
+        Assert.Equal(0.5f, s.PositionRatio, 3);
+        s.SetScrollY(200);
+        Assert.Equal(1f, s.PositionRatio, 3);
+    }
+
+    [Fact]
+    public void SetPositionRatio_IsInverseOfPositionRatio()
+    {
+        var s = new UiScrollable { ContentHeight = 300, ViewHeight = 100 };
+        s.SetPositionRatio(0.5f);
+        Assert.Equal(100, s.ScrollY);
+    }
+
+    [Fact]
+    public void ScrollByLines_AdvancesByLineHeight()
+    {
+        var s = new UiScrollable { ContentHeight = 1000, ViewHeight = 100, LineHeight = 16 };
+        s.ScrollByLines(-2);
+        Assert.Equal(0, s.ScrollY);
+        s.SetScrollY(50);
+        s.ScrollByLines(2);
+        Assert.Equal(82, s.ScrollY);
+    }
+
+    [Fact]
+    public void ScrollByPage_AdvancesByViewHeight()
+    {
+        var s = new UiScrollable { ContentHeight = 1000, ViewHeight = 100, LineHeight = 16 };
+        s.SetScrollY(200);
+        s.ScrollByPage(1);
+        Assert.Equal(300, s.ScrollY);
+    }
+}