fix(ui): retail scrollbar parity — button seating, full-track thumb, hover/pressed states

Owner report (2026-08-24): our scrollbar arrows pointed the wrong way,
the thumb vanished when there was nothing to scroll, and neither the
thumb nor the arrow buttons reacted to hover/press.

All three are one retail mechanism we had not ported:

1. Seating: UIElement_Scrollbar::UpdateScrollingArea @0x00470AA0 moves
   the INCREMENT designee (attribute 0x77) to the top/left corner and
   the DECREMENT designee (0x78) to the bottom/right, ignoring authored
   positions. The vertical base skin (0x10000455 in layout 0x2100003E)
   authors the DOWN-arrow decrement at Y=0 and the UP-arrow increment
   at Y=32 (live-DAT probed; sprite art visually verified from decoded
   PNGs), so our authored-Y ordering drew both arrows upside down.
   DatWidgetFactory now seats by designation; the hand-wired sites
   (CharacterStatController, ExternalContainerController, the
   Config/Vendor menu chrome) share the new RetailScrollbarChrome
   catalog instead of local constants.

2. Full-track thumb: UpdateLayout @0x004710d0 sizes the thumb from
   proportion attribute 0x88, which DEFAULTS to 1.0 — a content-fits
   bar shows a thumb filling the whole track; disabled only removes
   input and the page regions. Our draw skipped the thumb entirely on
   !HasOverflow.

3. States: every arrow button and thumb slice authors Normal (red gem /
   dark navy), Normal_rollover (amber gem / bright blue) and
   Normal_pressed (gold highlight / dark) media. The widget now tracks
   thumb hover and selects rollover media on hover and pressed media
   while dragging; the factory extracts the thumb-state media for both
   the 3-slice and single-sprite thumb shapes.

ScrollbarSkinLiveDatTests pins the designations and state media against
the installed DAT so a revision or importer regression fails loudly.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/AcDream.App.Tests/UI/Layout/ScrollbarSkinLiveDatTests.cs

@@ -0,0 +1,67 @@
+using AcDream.App.UI;
+using AcDream.App.UI.Layout;
+using DatReaderWriter;
+
+namespace AcDream.App.Tests.UI.Layout;
+
+/// <summary>
+/// 2026-08-24 pin: the base scrollbar skin (layout <c>0x2100003E</c>)
+/// authors the button DESIGNATIONS that drive retail's runtime seating
+/// (<c>UIElement_Scrollbar::UpdateScrollingArea @0x00470AA0</c> moves the
+/// increment designee to the top/left, the decrement designee to the
+/// bottom/right, ignoring authored positions). The vertical skin
+/// designates 0x77=0x10000072 (UP-arrow art) and 0x78=0x10000071
+/// (DOWN-arrow art) — seating by authored Y renders both arrows upside
+/// down, the owner-reported bug. Also pins the three-state media sets
+/// <see cref="RetailScrollbarChrome"/> mirrors, so a DAT revision or
+/// importer regression that loses a state fails loudly.
+/// </summary>
+[Trait("Lane", "InstalledDat")]
+public sealed class ScrollbarSkinLiveDatTests
+{
+    private static string DatDirectory =>
+        Path.Combine(
+            Environment.GetFolderPath(Environment.SpecialFolder.UserProfile),
+            "Documents", "Asheron's Call");
+
+    [InstalledDatFact]
+    public void VerticalBaseSkin_DesignatesUpArrowAsIncrement_WithThreeStateMedia()
+    {
+        using var dats = new DatCollection(
+            DatDirectory, DatReaderWriter.Options.DatAccessType.Read);
+        ElementInfo? tree = LayoutImporter.ImportInfos(dats, 0x2100003Eu);
+        Assert.NotNull(tree);
+
+        ElementInfo bar = Assert.Single(Flatten(tree!), e => e.Id == 0x10000455u);
+        Assert.True(bar.TryGetEffectiveProperty(0x77u, out UiPropertyValue inc));
+        Assert.True(bar.TryGetEffectiveProperty(0x78u, out UiPropertyValue dec));
+        Assert.Equal(0x10000072u, inc.UnsignedValue); // increment = UP arrow (authored Y=32)
+        Assert.Equal(0x10000071u, dec.UnsignedValue); // decrement = DOWN arrow (authored Y=0)
+
+        // Increment (top after seating): red-gem normal, amber rollover,
+        // highlight pressed — the RetailScrollbarChrome Up set.
+        ElementInfo up = Assert.Single(bar.Children, c => c.Id == 0x10000072u);
+        Assert.Equal(RetailScrollbarChrome.UpNormal, up.StateMedia["Normal"].File);
+        Assert.Equal(RetailScrollbarChrome.UpRollover, up.StateMedia["Normal_rollover"].File);
+        Assert.Equal(RetailScrollbarChrome.UpPressed, up.StateMedia["Normal_pressed"].File);
+        ElementInfo down = Assert.Single(bar.Children, c => c.Id == 0x10000071u);
+        Assert.Equal(RetailScrollbarChrome.DownNormal, down.StateMedia["Normal"].File);
+        Assert.Equal(RetailScrollbarChrome.DownRollover, down.StateMedia["Normal_rollover"].File);
+        Assert.Equal(RetailScrollbarChrome.DownPressed, down.StateMedia["Normal_pressed"].File);
+
+        // Thumb (structural child 1) slices each author the three states.
+        ElementInfo thumb = Assert.Single(bar.Children, c => c.Id == 1u);
+        ElementInfo mid = Assert.Single(thumb.Children, c => c.Id == 0x10000365u);
+        Assert.Equal(RetailScrollbarChrome.ThumbMidNormal, mid.StateMedia["Normal"].File);
+        Assert.Equal(RetailScrollbarChrome.ThumbMidRollover, mid.StateMedia["Normal_rollover"].File);
+        Assert.Equal(RetailScrollbarChrome.ThumbMidPressed, mid.StateMedia["Normal_pressed"].File);
+    }
+
+    private static IEnumerable<ElementInfo> Flatten(ElementInfo e)
+    {
+        yield return e;
+        foreach (var c in e.Children)
+            foreach (var d in Flatten(c))
+                yield return d;
+    }
+}