feat(ui): complete retail item drop branches

Give retained buttons a reusable item-drop seam, wire the toolbar backpack target, and port retail stack-merge legality, capacity clamping, wire dispatch, destination selection, and immediate shortcut rekey notice. Record the live ACE merge gate and keep split quantity under AP-101.

Co-Authored-By: Codex <codex@openai.com>

src/AcDream.Core/Items/StackMergePlanner.cs

@@ -0,0 +1,49 @@
+using System;
+
+namespace AcDream.Core.Items;
+
+public readonly record struct StackMergeItem(
+    uint ObjectId,
+    uint WeenieClassId,
+    int StackSize,
+    int MaxStackSize,
+    int TradeState);
+
+public readonly record struct StackMergePlan(uint SourceObjectId, uint TargetObjectId, uint Amount);
+
+/// <summary>
+/// Pure port of <c>ItemHolder::IsMergeAttemptLegal @ 0x00586F30</c> and the
+/// transfer-size branch in <c>ItemHolder::AttemptMerge @ 0x005878F0</c>.
+/// </summary>
+public static class StackMergePlanner
+{
+    public static StackMergePlan? Plan(
+        in StackMergeItem source,
+        in StackMergeItem target,
+        bool readyForInventoryRequest,
+        int requestedAmount)
+    {
+        if (!readyForInventoryRequest
+            || source.ObjectId == 0
+            || target.ObjectId == 0
+            || source.ObjectId == target.ObjectId
+            || source.MaxStackSize <= 1
+            || target.MaxStackSize <= 1
+            || source.TradeState == 1
+            || target.TradeState == 1
+            || source.WeenieClassId != target.WeenieClassId)
+            return null;
+
+        int targetSize = Math.Max(1, target.StackSize);
+        int available = target.MaxStackSize - targetSize;
+        if (available <= 0)
+            return null;
+
+        int sourceSize = Math.Max(1, source.StackSize);
+        int desired = requestedAmount > 0 ? Math.Min(requestedAmount, sourceSize) : sourceSize;
+        uint transfer = (uint)Math.Min(desired, available);
+        return transfer == 0
+            ? null
+            : new StackMergePlan(source.ObjectId, target.ObjectId, transfer);
+    }
+}