feat(ui): port retail selected-stack quantity

Bind the authored stack count entry and horizontal slider to one Core split-quantity owner, preserve retail count-first naming and exact 1000-step rounding, refresh on stack changes, and consume the selected amount during merges. Conformance covers the production DAT fixture and retained pointer/focus paths.

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

tests/AcDream.App.Tests/UI/Layout/SelectedObjectControllerTests.cs

@@ -4,6 +4,7 @@ using System.Linq;
 using System.Numerics;
 using AcDream.App.UI;
 using AcDream.App.UI.Layout;
+using AcDream.Core.Items;
 using AcDream.Core.Selection;
 using Xunit;
 
@@ -59,6 +60,14 @@ public class SelectedObjectControllerTests
         dict[SelectedObjectController.HealthMeterId] = healthMeterEl;
         root.AddChild(healthMeterEl);
 
+        var stackEntry = new UiField { Width = 50, Height = 14, Visible = true };
+        dict[SelectedObjectController.StackSizeEntryId] = stackEntry;
+        root.AddChild(stackEntry);
+
+        var stackSlider = new UiScrollbar { Width = 90, Height = 14, Visible = true };
+        dict[SelectedObjectController.StackSizeSliderId] = stackSlider;
+        root.AddChild(stackSlider);
+
         return (new ImportedLayout(root, dict), nameEl, overlayEl, healthMeterEl);
     }
 
@@ -67,7 +76,9 @@ public class SelectedObjectControllerTests
     private sealed class Harness
     {
         public readonly SelectionState Selection = new();
+        public readonly StackSplitQuantityState SplitQuantity = new();
         public Action<uint, float>? HealthHandler;
+        public Action<ClientObject>? ObjectUpdatedHandler;
         public readonly List<uint> QueryHealthCalls = new();
 
         public readonly Dictionary<uint, bool>   HealthTargetMap = new();
@@ -100,7 +111,13 @@ public class SelectedObjectControllerTests
                 hasHealth:       g => HasHealthMap.TryGetValue(g, out var v) && v,
                 stackSize:       g => StackMap.TryGetValue(g, out var v) ? v : 0u,
                 sendQueryHealth: g => QueryHealthCalls.Add(g),
-                datFont:         datFont);
+                datFont:         datFont,
+                splitQuantity:   SplitQuantity,
+                subscribeObjectUpdated: h => ObjectUpdatedHandler = h,
+                unsubscribeObjectUpdated: h =>
+                {
+                    if (ObjectUpdatedHandler == h) ObjectUpdatedHandler = null;
+                });
     }
 
     // ── B1: Bind initialisation ──────────────────────────────────────────────
@@ -247,6 +264,58 @@ public class SelectedObjectControllerTests
         Assert.False(healthMeterEl.Visible);
     }
 
+    [Fact]
+    public void SelectStackedItem_showsRetailCountEntryAndSlider_atFullStack()
+    {
+        const uint Guid = 0xBB03u;
+        var (layout, nameEl, _, _) = FakeLayout();
+        var h = new Harness();
+        h.NameMap[Guid] = "Healing Kits";
+        h.StackMap[Guid] = 17u;
+        h.Bind(layout);
+
+        h.FireSelection(Guid);
+
+        var entry = Assert.IsType<UiField>(layout.FindElement(SelectedObjectController.StackSizeEntryId));
+        var slider = Assert.IsType<UiScrollbar>(layout.FindElement(SelectedObjectController.StackSizeSliderId));
+        Assert.True(entry.Visible);
+        Assert.True(slider.Visible);
+        Assert.Equal("17", entry.Text);
+        Assert.Equal(17u, h.SplitQuantity.Value);
+        Assert.Equal(17u, h.SplitQuantity.Maximum);
+        Assert.Equal(1f, slider.ScalarValue!());
+        Assert.Equal("17 Healing Kits", nameEl.Children.OfType<UiText>().Single().LinesProvider().Single().Text);
+
+        slider.ScalarChanged!(0.5f);
+        Assert.Equal(9u, h.SplitQuantity.Value);
+        Assert.Equal("9", entry.Text);
+
+        entry.SetText("999");
+        entry.Submit();
+        Assert.Equal(17u, h.SplitQuantity.Value);
+        Assert.Equal("17", entry.Text);
+    }
+
+    [Fact]
+    public void StackSizeUpdate_refreshesSelectedControls_andHidesThemAtOne()
+    {
+        const uint Guid = 0xBB04u;
+        var (layout, _, _, _) = FakeLayout();
+        var h = new Harness();
+        h.NameMap[Guid] = "Healing Kits";
+        h.StackMap[Guid] = 5u;
+        h.Bind(layout);
+        h.FireSelection(Guid);
+
+        h.StackMap[Guid] = 1u;
+        h.ObjectUpdatedHandler!(new ClientObject { ObjectId = Guid, StackSize = 1 });
+
+        Assert.False(layout.FindElement(SelectedObjectController.StackSizeEntryId)!.Visible);
+        Assert.False(layout.FindElement(SelectedObjectController.StackSizeSliderId)!.Visible);
+        Assert.Equal(1u, h.SplitQuantity.Value);
+        Assert.Equal(1u, h.SplitQuantity.Maximum);
+    }
+
     // ── H3: Non-health target (friendly NPC / scenery / Door) ───────────────
 
     [Fact]