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>

tests/AcDream.Core.Tests/Items/StackMergePlannerTests.cs

@@ -0,0 +1,60 @@
+using AcDream.Core.Items;
+
+namespace AcDream.Core.Tests.Items;
+
+public sealed class StackMergePlannerTests
+{
+    [Fact]
+    public void MatchingStacks_TransfersRequestedAmount()
+    {
+        StackMergePlan? plan = StackMergePlanner.Plan(
+            Item(1, size: 40), Item(2, size: 20),
+            readyForInventoryRequest: true, requestedAmount: 15);
+
+        Assert.Equal(new StackMergePlan(1, 2, 15), plan);
+    }
+
+    [Fact]
+    public void Transfer_IsLimitedByDestinationCapacity()
+    {
+        StackMergePlan? plan = StackMergePlanner.Plan(
+            Item(1, size: 40), Item(2, size: 95),
+            readyForInventoryRequest: true, requestedAmount: 40);
+
+        Assert.Equal(new StackMergePlan(1, 2, 5), plan);
+    }
+
+    [Fact]
+    public void ZeroRequestedAmount_UsesWholeSourceStack()
+    {
+        StackMergePlan? plan = StackMergePlanner.Plan(
+            Item(1, size: 7), Item(2, size: 20),
+            readyForInventoryRequest: true, requestedAmount: 0);
+
+        Assert.Equal(new StackMergePlan(1, 2, 7), plan);
+    }
+
+    [Theory]
+    [InlineData(false, 10u, 10, 0, 100, 100)]
+    [InlineData(true, 11u, 10, 0, 100, 100)]
+    [InlineData(true, 10u, 10, 1, 100, 100)]
+    [InlineData(true, 10u, 10, 0, 1, 100)]
+    [InlineData(true, 10u, 10, 0, 100, 1)]
+    [InlineData(true, 10u, 100, 0, 100, 100)]
+    public void IllegalMerge_ReturnsNoPlan(
+        bool ready,
+        uint targetWcid,
+        int targetSize,
+        int sourceTradeState,
+        int sourceMax,
+        int targetMax)
+    {
+        var source = new StackMergeItem(1, 10, 10, sourceMax, sourceTradeState);
+        var target = new StackMergeItem(2, targetWcid, targetSize, targetMax, 0);
+
+        Assert.Null(StackMergePlanner.Plan(source, target, ready, requestedAmount: 10));
+    }
+
+    private static StackMergeItem Item(uint id, int size)
+        => new(id, WeenieClassId: 10, StackSize: size, MaxStackSize: 100, TradeState: 0);
+}