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/ShortcutDropPlanner.cs

@@ -92,8 +92,8 @@ public static class ShortcutDropPlanner
 
     /// <summary>
     /// Plan the toolbar side of <c>RecvNotice_FullMergingItem @ 0x004BE9B0</c>.
-    /// The normal retail invariant is one object shortcut; all matching records are
-    /// rekeyed here so a malformed duplicate cannot retain a destroyed stack id.
+    /// <c>CreateShortcutToItem</c> removes an existing destination shortcut before
+    /// placing it into the source shortcut's former slot.
     /// </summary>
     public static ShortcutMutation[] PlanFullStackMerge(
         IReadOnlyList<ShortcutEntry?> slots,
@@ -104,15 +104,21 @@ public static class ShortcutDropPlanner
         if (slots.Count != ShortcutStore.SlotCount || oldObjectId == 0 || newObjectId == 0)
             return Array.Empty<ShortcutMutation>();
 
-        var mutations = new List<ShortcutMutation>();
+        var state = new ShortcutEntry?[ShortcutStore.SlotCount];
         for (int slot = 0; slot < slots.Count; slot++)
         {
-            if (VisibleEntry(slots[slot]) is not { } entry || entry.ObjectId != oldObjectId)
-                continue;
-
-            mutations.Add(ShortcutMutation.Remove(slot));
-            mutations.Add(ShortcutMutation.Add(entry with { ObjectId = newObjectId }));
+            state[slot] = slots[slot];
         }
+
+        var mutations = new List<ShortcutMutation>(3);
+        int sourceSlot = FindFirstVisibleObject(state, oldObjectId);
+        if (sourceSlot < 0)
+            return Array.Empty<ShortcutMutation>();
+
+        ShortcutEntry sourceEntry = state[sourceSlot]!.Value;
+        RemoveVisibleAt(state, sourceSlot, mutations);
+        RemoveFirstVisibleObject(state, newObjectId, mutations);
+        Add(state, sourceEntry with { ObjectId = newObjectId }, mutations);
         return mutations.ToArray();
     }
 
@@ -149,6 +155,14 @@ public static class ShortcutDropPlanner
         }
     }
 
+    private static int FindFirstVisibleObject(ShortcutEntry?[] state, uint objectId)
+    {
+        for (int slot = 0; slot < state.Length; slot++)
+            if (VisibleEntry(state[slot]) is { } entry && entry.ObjectId == objectId)
+                return slot;
+        return -1;
+    }
+
     private static void Add(
         ShortcutEntry?[] state,
         ShortcutEntry entry,

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);
+    }
+}