fix: trade gate round 1 - the Use-on-player route, authored grid cells,

and [trade] seam probes

- Use on a selected player did nothing: the world Use path
  (SelectionInteractionController.RequestUse) dispatches 0x0036 directly
  and NEVER consults DetermineUseResult - lane C's "both paths already
  classified" claim described the POLICY's capability, not a live
  caller (the verify-subagent-claims lesson, again). Retail's
  CPlayerSystem::UsingItem @0x00562F70 consults it and routes result 5
  (another PLAYER) to AttemptToOpenTradeNegotiations @0x0056DEE0, peace
  mode only, with no Use send. Ported as
  ItemInteractionController.TryOpenSecureTradeWithPlayer, called at
  RequestUse entry.
- The broken window: both grids rendered their raw authored strip art
  with no cell layout (the gold-ring tile was the naked track). They now
  get the vendor strips' exact single-row 32px config + the authored
  empty-slot art resolved through ItemListCellTemplate (cell-template
  attr 0x1000000E -> 0x1000033A).
- Drag-onto-player "nothing happens": the policy chain reads correct
  end-to-end in code (pick -> PlaceIn3D -> StartSecureTrade -> event ->
  cmd), so TEMPORARY [trade] probe lines now bracket every seam
  (drag-release pick, policy arm, controller request, use-on-player) -
  the next gate log pinpoints the break if it persists. Stripped once
  the two-client gate passes.

App suite 4,990/3 skips.

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

src/AcDream.App/UI/RetailUiRuntime.cs

@@ -3489,6 +3489,8 @@ public sealed class RetailUiRuntime : IDisposable
         }
 
         ImportedLayout? layout;
+        uint selfEmptySlotSprite;
+        uint partnerEmptySlotSprite;
         lock (_bindings.Assets.DatLock)
         {
             layout = LayoutImporter.Import(
@@ -3498,6 +3500,16 @@ public sealed class RetailUiRuntime : IDisposable
                 _bindings.Assets.ResolveSprite,
                 _bindings.Assets.DefaultFont,
                 _bindings.Assets.ResolveFont);
+            // The authored empty-slot art for each grid (cell-template attr
+            // 0x1000000E → 0x1000033A) — same resolution the vendor uses.
+            selfEmptySlotSprite = ItemListCellTemplate.ResolveEmptySprite(
+                _bindings.Assets.Dats,
+                Layout.SecureTradeUiController.LayoutId,
+                Layout.SecureTradeUiController.SelfListId);
+            partnerEmptySlotSprite = ItemListCellTemplate.ResolveEmptySprite(
+                _bindings.Assets.Dats,
+                Layout.SecureTradeUiController.LayoutId,
+                Layout.SecureTradeUiController.PartnerListId);
         }
         if (layout is null)
         {
@@ -3529,7 +3541,9 @@ public sealed class RetailUiRuntime : IDisposable
                     {
                         if (visible) Host.ShowWindow(WindowNames.SecureTrade);
                         else Host.HideWindow(WindowNames.SecureTrade);
-                    }));
+                    },
+                    SelfEmptySlotSprite: selfEmptySlotSprite,
+                    PartnerEmptySlotSprite: partnerEmptySlotSprite));
         if (controller is null)
         {
             Console.WriteLine("[M4] secure trade: required authored grids are missing.");