feat(ui): Slice 5.4 — the authored vendor browse panel (LayoutDesc 0x21000012)

The vendor window is retail's own: LayoutDesc 0x21000012, root
0x100000B7, found by enumerating all 101 layouts for the one
containing both known tab controls and clinched by the root's Type
0x10000017 — the literal UIElement::RegisterElementClass id for
gmVendorUI (pc:202075). Discovery evidence and the D0 read live in
the research doc's new §B.4.

D0 corrected two assumptions: retail's category "tabs" are a UiMenu
DROPDOWN fed by a hardcoded 18-row ordered category table (ported
bit-for-bit against our ItemType enum; list always scoped to exactly
one category, first-present wins, selection preserved across refresh
per retail's clamp), and the layout authors THREE tabs — Items
(browse, this slice), Buying and Selling (staged-transaction review,
Slice 6) — decision 4's "browse/Buy tab" names the Items tab retail's
mode-2 OpenTab opens. The non-default tabs render and switch pages
but stay inert, fenced in comments.

VendorUiController mounts Items: category dropdown, icon-cell item
row with the retained scrollbar, per-unit retail pricing via
VendorPricing.SellPrice (the vendor-stock path VendorProfile::
VendorSellPrice feeds), name/cost on selection. The panel is a pure
projection of VendorState — opens on populate, closes on clear; the
close button's VendorState.Close() is its only permitted mutation.
Nothing on the wire.

AP-110 narrowed (vendor leaves the absent-panels list); AP-161 files
the precise Slice-6 remainder (Buying/Selling unwired, Buy/Add
buttons, InqAcceptability). Twelve controller tests on a real-dat
fixture. Clean-room complete solution: 11,323 passed / 4 skipped /
0 failed.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/AcDream.App/UI/RetailUiRuntime.cs

@@ -168,6 +168,10 @@ public sealed record AppraisalRuntimeBindings(
     Action<uint, string> SendSetInscription,
     Action<string> DisplaySystemMessage);
 
+public sealed record VendorRuntimeBindings(
+    VendorState State,
+    Func<ItemType, uint, uint, uint, uint, uint> ResolveIcon);
+
 public sealed record RetailUiRuntimeBindings(
     UiHost Host,
     RetailUiAssets Assets,
@@ -184,6 +188,7 @@ public sealed record RetailUiRuntimeBindings(
     CharacterRuntimeBindings Character,
     InventoryRuntimeBindings Inventory,
     ExternalContainerRuntimeBindings ExternalContainer,
+    VendorRuntimeBindings Vendor,
     RetailUiCursorBindings Cursor,
     ConfirmationRuntimeBindings Confirmations,
     AppraisalRuntimeBindings Appraisal,
@@ -255,6 +260,7 @@ public sealed class RetailUiRuntime : IDisposable
         MountPlugins();
         MountInventory();
         MountExternalContainer();
+        MountVendor();
         MountItemCooldowns();
         Host.WindowManager.WindowVisibilityChanged += OnWindowVisibilityChanged;
         BindToolbarPanelButtons();
@@ -272,6 +278,7 @@ public sealed class RetailUiRuntime : IDisposable
                     WindowNames.Combat,
                     WindowNames.JumpPowerbar,
                     WindowNames.ExternalContainer,
+                    WindowNames.Vendor,
                 ]);
         }
 
@@ -322,6 +329,7 @@ public sealed class RetailUiRuntime : IDisposable
     public InventoryController? InventoryPanelController { get; private set; }
     public RetailDialogFactory? DialogFactory { get; private set; }
     public ExternalContainerController? ExternalContainerController { get; private set; }
+    public VendorUiController? VendorController { get; private set; }
 
     public static RetailUiRuntime Mount(RetailUiRuntimeBindings bindings)
     {
@@ -1897,6 +1905,75 @@ public sealed class RetailUiRuntime : IDisposable
             "[D.2b] retail external-container strip mounted from LayoutDesc 0x21000008.");
     }
 
+    /// <summary>
+    /// Slice 5.4: the retail vendor "Items" browse tab (LayoutDesc
+    /// 0x21000012, root 0x100000B7 — see
+    /// docs/research/2026-08-08-slice5-vendor-browse-research.md §B.3/§B.4
+    /// for the discovery + D0 category-filter read). Two-step mount
+    /// (Mount then Bind then AttachController) matches
+    /// <see cref="MountExternalContainer"/>: the controller needs the
+    /// <see cref="RetailWindowHandle"/> to Show/Hide itself as
+    /// <see cref="VendorState"/> opens/closes, so the handle must exist
+    /// before the controller is constructed.
+    /// </summary>
+    private void MountVendor()
+    {
+        ImportedLayout? layout;
+        lock (_bindings.Assets.DatLock)
+        {
+            layout = LayoutImporter.Import(
+                _bindings.Assets.Dats,
+                VendorUiController.LayoutId,
+                VendorUiController.RootId,
+                _bindings.Assets.ResolveSprite,
+                _bindings.Assets.DefaultFont,
+                _bindings.Assets.ResolveFont);
+        }
+        if (layout is null)
+        {
+            Console.WriteLine("[M4] vendor: LayoutDesc 0x21000012 root 0x100000B7 not found.");
+            return;
+        }
+
+        UiElement root = layout.Root;
+        RetailWindowHandle handle = RetailWindowFrame.Mount(
+            Host.Root,
+            root,
+            _bindings.Assets.ResolveSprite,
+            new RetailWindowFrame.Options
+            {
+                WindowName = WindowNames.Vendor,
+                // Root 0x100000B7 authors only the tiled center surface
+                // (backdrop element 0x1000008D provides its own fill); the
+                // common floating-window bevel surrounds it — same shape as
+                // ExternalContainerController's 0x21000008/0x10000063.
+                Chrome = RetailWindowChrome.NineSlice,
+                Left = root.Left,
+                Top = root.Top,
+                ContentWidth = root.Width,
+                ContentHeight = root.Height,
+                MinWidth = root.Width,
+                MinHeight = root.Height,
+                Visible = false,
+                ResizeX = false,
+                ResizeY = false,
+                ConstrainDragToParent = true,
+                ConstrainResizeToParent = true,
+                DrawChromeCenter = false,
+            });
+
+        VendorRuntimeBindings b = _bindings.Vendor;
+        VendorController = VendorUiController.Bind(layout, b.State, handle, b.ResolveIcon);
+        if (VendorController is null)
+        {
+            Console.WriteLine("[M4] vendor: required authored controls are missing.");
+            return;
+        }
+
+        Host.WindowManager.AttachController(WindowNames.Vendor, VendorController);
+        Console.WriteLine("[M4] retail vendor browse panel mounted from LayoutDesc 0x21000012.");
+    }
+
     private void MountItemCooldowns()
     {
         ItemCooldownAssets? assets;