feat(ui): D.2b inventory finish — bind contents-grid gutter scrollbar

InventoryController binds 0x100001C7 (factory Type-11 UiScrollbar) to the
contents grid's UiScrollable + the shared 0x2100003E scrollbar sprites,
mirroring ChatWindowController. The grid now scrolls instead of overflowing.

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

src/AcDream.App/UI/Layout/InventoryController.cs

@@ -20,6 +20,16 @@ public sealed class InventoryController
     public const uint BurdenTextId      = 0x100001D8u;  // gmBackpackUI m_burdenText ("%d%%")
     public const uint BurdenCaptionId   = 0x100001D7u;  // "Burden"
     public const uint ContentsCaptionId = 0x100001C5u;  // "Contents of Backpack"
+    public const uint ContentsScrollbarId = 0x100001C7u; // gm3DItemsUI gutter scrollbar (right of the grid)
+
+    // Scrollbar chrome from base layout 0x2100003E (shared with the chat scrollbar; see
+    // ChatWindowController). Both inventory + chat scrollbars inherit this base.
+    private const uint ScrollTrackSprite  = 0x06004C5Fu;
+    private const uint ScrollThumbSprite  = 0x06004C63u;  // 3-slice middle tile
+    private const uint ScrollThumbTop     = 0x06004C60u;  // 3-slice top cap
+    private const uint ScrollThumbBot     = 0x06004C66u;  // 3-slice bottom cap
+    private const uint ScrollUpSprite     = 0x06004C6Cu;  // up arrow (top button)
+    private const uint ScrollDownSprite   = 0x06004C69u;  // down arrow (bottom button)
 
     // 3D-items grid: 192x96 → 6 cols x 3 rows of the 32x32 UIItem cell (template 0x21000037).
     private const int   ContentsColumns = 6;
@@ -66,6 +76,21 @@ public sealed class InventoryController
             _contentsGrid.CellWidth  = CellPx;
             _contentsGrid.CellHeight = CellPx;
         }
+
+        // Bind the gutter scrollbar to the contents grid's scroll model (the factory built
+        // 0x100001C7 as a bare Type-11 UiScrollbar; wire it like ChatWindowController does).
+        if (_contentsGrid is not null
+            && layout.FindElement(ContentsScrollbarId) is UiScrollbar bar)
+        {
+            bar.Model          = _contentsGrid.Scroll;
+            bar.SpriteResolve  = _contentsGrid.SpriteResolve;   // chrome resolve from the factory ctor
+            bar.TrackSprite    = ScrollTrackSprite;
+            bar.ThumbSprite    = ScrollThumbSprite;
+            bar.ThumbTopSprite = ScrollThumbTop;
+            bar.ThumbBotSprite = ScrollThumbBot;
+            bar.UpSprite       = ScrollUpSprite;
+            bar.DownSprite     = ScrollDownSprite;
+        }
         if (_containerList is not null)
         {
             _containerList.Columns    = 1;

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

@@ -22,6 +22,7 @@ public class InventoryControllerTests
     private const uint BurdenText      = 0x100001D8u;
     private const uint BurdenCaption   = 0x100001D7u;
     private const uint ContentsCaption = 0x100001C5u;
+    private const uint ContentsScrollbar = 0x100001C7u;
 
     private static (ImportedLayout layout, UiItemList grid, UiItemList containers,
                     UiItemList top, UiMeter meter, UiElement burdenText,
@@ -34,15 +35,16 @@ public class InventoryControllerTests
         var burdenText  = new TestElement { Width = 36,  Height = 15 };
         var burdenCap   = new TestElement { Width = 36,  Height = 15 };
         var contentsCap = new TestElement { Width = 192, Height = 15 };
+        var scrollbar   = new UiScrollbar { Width = 16,  Height = 96 };
         var root        = new TestElement { Width = 300, Height = 362 };
         root.AddChild(grid); root.AddChild(containers); root.AddChild(top);
         root.AddChild(meter); root.AddChild(burdenText); root.AddChild(burdenCap);
-        root.AddChild(contentsCap);
+        root.AddChild(contentsCap); root.AddChild(scrollbar);
         var byId = new Dictionary<uint, UiElement>
         {
             [ContentsGrid]    = grid, [ContainerList] = containers, [TopContainer] = top,
             [BurdenMeter]     = meter, [BurdenText] = burdenText, [BurdenCaption] = burdenCap,
-            [ContentsCaption] = contentsCap,
+            [ContentsCaption] = contentsCap, [ContentsScrollbar] = scrollbar,
         };
         return (new ImportedLayout(root, byId), grid, containers, top, meter,
                 burdenText, burdenCap, contentsCap);
@@ -192,6 +194,15 @@ public class InventoryControllerTests
         Assert.Equal(0.2f, meter.Fill() ?? -1f, 3);     // 9000/15000/3
     }
 
+    [Fact]
+    public void Contents_grid_scrollbar_binds_to_the_grid_scroll_model()
+    {
+        var (layout, grid, _, _, _, _, _, _) = BuildLayout();
+        Bind(layout, new ClientObjectTable());
+        var bar = (UiScrollbar)layout.FindElement(ContentsScrollbar)!;
+        Assert.Same(grid.Scroll, bar.Model);            // the bar drives the grid's scroll
+    }
+
     // Reads the text of the UiText caption child attached by the controller.
     private static string CaptionText(UiElement host)
     {