feat: secure trade with other players - wire, RuntimeTradeState, the

authored gmSecureTradeUI window, and both retail open paths

Three-lane research first (docs/research/2026-08-14-trade-lane{A,B,C}):
retail gmSecureTradeUI decode, the byte-exact ACE/decomp/holtburger
three-way wire agreement, and the acdream seam map (which found both
open paths ALREADY classified by the ported policy - OpenSecureTrade on
Use-a-player, StartSecureTrade on drag-item-onto-player with the
DragItemOnPlayerOpensSecureTrade option - dead-ending at a stub toast).

- Core.Net: TradeRequests builders (0x1F6-0x204, retail's CM_Trade
  senders byte-checked against ACE's readers; the ACE-discarded
  AcceptTrade echo carries zero-count item lists - AD-94), corrected +
  completed inbound parsers (0x1FD-0x208; the old AddToTrade parser
  missed the SIDE dword, TradeFailure missed the reason), delegate-hole
  registrars, six WorldSession sends. 10 golden-byte tests.
- Runtime: RuntimeTradeState, the third sibling J-owner (fellowship/
  allegiance shape): session-scoped, clears at generation reset (new
  stage Trade=14), staged teardown stage 11 (Identity/EntityObjects
  shift 12/13, TeardownStageCount 14 - the FA2-era per-stage-flag test
  caught the mapping exactly as designed), combined ownership ledger,
  event routing with ACE's wrong-initiator RegisterTrade landmine
  honored (partner = whichever guid is not mine). 7 conformance tests.
- App: SecureTradeUiController binds the dedicated authored LayoutDesc
  0x2100000D (root 0x1000007A - gmSecureTradeUI::PostInit's exact ids):
  partner name/status/count/grid, the authored 'Trade' accept toggle
  (accept <-> decline withdraw), 'Clear All' (ACE clears BOTH sides -
  surfaced honestly), the X close, drop-on-your-grid staging, per-mode
  accept cues (partner icon's authored Highlight state + Trade button
  Selected latch). Mounted via the vendor recipe (nine-slice chrome,
  hidden until RegisterTrade). ItemInteractionController's two policy
  arms now raise SecureTradeRequested instead of the stub toast; the
  drag path queues the dragged item until the window registers
  (ClientTradeSystem::AttemptToTradeItem @0x0056DF80's shape).

Register: AD-94 (accept-echo zero-count lists), AD-95 (numeric-only
count texts pending template verification).

Suites: App 4,990/3, Core.Net 905, Runtime 1,626 - all green. The
panel itself is user-gate acceptance (two-client connected trade), the
#372-class lesson: fixture-green alone is not acceptance for a mount.

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

src/AcDream.App/UI/ItemInteractionController.cs

@@ -182,6 +182,16 @@ public sealed class ItemInteractionController : IDisposable
 
     public event Action? StateChanged;
 
+    /// <summary>
+    /// Retail's two secure-trade open paths surface here for the trade UI:
+    /// (partnerGuid, itemGuid) — itemGuid 0 for Use-on-player
+    /// (<c>DetermineUseResult @ 0x00588460</c> result 5), non-zero for
+    /// drag-item-onto-player with the DragItemOnPlayerOpensSecureTrade
+    /// option (<c>AttemptPlaceIn3D @ 0x00588600</c>). The subscriber
+    /// (SecureTradeUiController) owns the open/stage sequencing.
+    /// </summary>
+    public event Action<uint, uint>? SecureTradeRequested;
+
     /// <summary>
     /// Retail <c>CM_Item::SendNotice_ShowPendingInPlayer</c>: the inventory
     /// panel inserts a waiting projection before the pickup request is sent.
@@ -1139,6 +1149,14 @@ public sealed class ItemInteractionController : IDisposable
                     if (!string.IsNullOrWhiteSpace(action.Message))
                         _toast?.Invoke(action.Message);
                     break;
+                case ItemPolicyActionKind.OpenSecureTrade:
+                    // Use-on-player (ItemHolder::DetermineUseResult
+                    // @ 0x00588460 result 5 → ClientTradeSystem::
+                    // AttemptToOpenTradeNegotiations @ 0x0056DEE0). The
+                    // action's ObjectId IS the target player.
+                    SecureTradeRequested?.Invoke(action.ObjectId, 0u);
+                    acted |= SecureTradeRequested is not null;
+                    break;
                 default:
                     _auxiliaryAction?.Invoke(action);
                     PolicyActionRequested?.Invoke(action);
@@ -1161,6 +1179,13 @@ public sealed class ItemInteractionController : IDisposable
         {
             switch (action.Kind)
             {
+                case ItemPolicyActionKind.StartSecureTrade:
+                    // Drag-item-onto-player with DragItemOnPlayerOpensSecureTrade
+                    // (ItemHolder::AttemptPlaceIn3D @ 0x00588600's option branch
+                    // → ClientTradeSystem::AttemptToTradeItem @ 0x0056DF80).
+                    // ObjectId = the dragged item, TargetId = the player.
+                    SecureTradeRequested?.Invoke(action.TargetId, action.ObjectId);
+                    break;
                 case ItemPolicyActionKind.DropToWorld:
                     TryDispatchInventoryRequest(
                         InventoryRequestKind.DropToWorld,