feat(ui): port favorite spell bar overflow

Import the retail arrow-only spell bar scrollbar from LayoutDesc, preserve authored end-button extents and HideDisabled behavior in the shared retained widget, and bind each favorite list to its sole horizontal pixel-scroll model. Match retail selection exposure for off-screen spells and pin the behavior with real-fixture conformance tests.

Document the six-slice world-interaction completion program as the active pre-M4 work order.

Co-authored-by: Codex <codex@openai.com>

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

@@ -128,6 +128,60 @@ public class UiScrollbarTests
         Assert.True(bar.OnEvent(new UiEvent(0u, bar, UiEventType.MouseUp, Data1: 144)));
     }
 
+    [Fact]
+    public void HorizontalModel_UsesAuthoredArrowExtentsAndOneCellStep()
+    {
+        var model = new UiScrollable
+        {
+            ContentHeight = 640,
+            ViewHeight = 320,
+            LineHeight = 32,
+        };
+        model.SetScrollY(64);
+        var bar = new UiScrollbar
+        {
+            Width = 160f,
+            Height = 36f,
+            Horizontal = true,
+            Model = model,
+            DecrementButtonExtent = 23f,
+            IncrementButtonExtent = 29f,
+        };
+
+        Assert.True(bar.OnEvent(new UiEvent(
+            0u, bar, UiEventType.MouseDown, Data1: 22)));
+        Assert.Equal(32, model.ScrollY);
+
+        Assert.True(bar.OnEvent(new UiEvent(
+            0u, bar, UiEventType.MouseDown, Data1: 131)));
+        Assert.Equal(64, model.ScrollY);
+    }
+
+    [Fact]
+    public void ModelWithoutOverflow_IsDisabledAndHideDisabledSuppressesPresentation()
+    {
+        var model = new UiScrollable
+        {
+            ContentHeight = 320,
+            ViewHeight = 320,
+            LineHeight = 32,
+        };
+        var bar = new UiScrollbar
+        {
+            Width = 160f,
+            Height = 36f,
+            Horizontal = true,
+            Model = model,
+            HideWhenDisabled = true,
+        };
+
+        Assert.True(bar.IsModelDisabled);
+        Assert.False(bar.IsPresentationVisible);
+        Assert.False(bar.OnEvent(new UiEvent(
+            0u, bar, UiEventType.MouseDown, Data1: 159)));
+        Assert.Equal(0, model.ScrollY);
+    }
+
     [Theory]
     [InlineData(0f,   0f,   0f)]
     [InlineData(0.5f, 0f,  50f)]