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>

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

@@ -136,6 +136,8 @@ public sealed class RetailLayoutFixtureGenerator
                 "examine_row_2100006B_10000166.json"),
             (AppraisalUiController.LayoutId, SpellExamineComponentTemplateFactory.TemplateId,
                 "examine_component_2100006B_1000032E.json"),
+            (VendorUiController.LayoutId, VendorUiController.RootId,
+                "vendor_21000012_100000B7.json"),
         })
         {
             ElementInfo? panelPart = LayoutImporter.ImportInfos(dats, layoutId, rootId);