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.Core.Tests/Items/StackSplitQuantityStateTests.cs

@@ -0,0 +1,58 @@
+using AcDream.Core.Items;
+using Xunit;
+
+namespace AcDream.Core.Tests.Items;
+
+public sealed class StackSplitQuantityStateTests
+{
+    [Fact]
+    public void Reset_initializesToFullStack_andClampsZeroMaximumToOne()
+    {
+        var state = new StackSplitQuantityState();
+
+        state.Reset(17u);
+        Assert.Equal(17u, state.Value);
+        Assert.Equal(17u, state.Maximum);
+        Assert.Equal(1f, state.Ratio);
+
+        state.Reset(0u);
+        Assert.Equal(1u, state.Value);
+        Assert.Equal(1u, state.Maximum);
+    }
+
+    [Theory]
+    [InlineData(null, 1u)]
+    [InlineData("", 1u)]
+    [InlineData("invalid", 1u)]
+    [InlineData("0", 1u)]
+    [InlineData("7", 7u)]
+    [InlineData("999", 10u)]
+    public void Text_usesWcstoulStyleParse_thenClamps(string? text, uint expected)
+    {
+        var state = new StackSplitQuantityState();
+        state.Reset(10u);
+
+        state.SetFromText(text);
+
+        Assert.Equal(expected, state.Value);
+    }
+
+    [Theory]
+    [InlineData(-1f, 1u)]
+    [InlineData(0f, 1u)]
+    [InlineData(0.099f, 1u)]
+    [InlineData(0.1f, 2u)]
+    [InlineData(0.5f, 6u)]
+    [InlineData(0.999f, 10u)]
+    [InlineData(1f, 10u)]
+    [InlineData(2f, 10u)]
+    public void Slider_matchesRetailThousandStepConversion(float ratio, uint expected)
+    {
+        var state = new StackSplitQuantityState();
+        state.Reset(10u);
+
+        state.SetFromSliderRatio(ratio);
+
+        Assert.Equal(expected, state.Value);
+    }
+}