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/UiScrollbarTests.cs

@@ -78,4 +78,26 @@ public class UiScrollbarTests
         Assert.Equal(100f, h, 3f);
         Assert.Equal(16f, y, 3f); // travel = 0 → y = trackTop
     }
+
+    [Fact]
+    public void HorizontalScalar_clickAndDrag_updatesNormalizedValue()
+    {
+        float value = 1f;
+        var bar = new UiScrollbar
+        {
+            Width = 90f,
+            Height = 14f,
+            Horizontal = true,
+            ScalarValue = () => value,
+            ScalarChanged = next => value = next,
+        };
+
+        Assert.True(bar.OnEvent(new UiEvent(0u, bar, UiEventType.MouseDown, Data1: 8)));
+        Assert.Equal(0f, value, 3);
+
+        Assert.True(bar.OnEvent(new UiEvent(0u, bar, UiEventType.MouseMove, Data1: 45)));
+        Assert.Equal(0.5f, value, 3);
+
+        Assert.True(bar.OnEvent(new UiEvent(0u, bar, UiEventType.MouseUp, Data1: 45)));
+    }
 }