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>

src/AcDream.Core/Items/StackSplitQuantityState.cs

@@ -0,0 +1,62 @@
+using System;
+using System.Globalization;
+
+namespace AcDream.Core.Items;
+
+/// <summary>
+/// Shared owner for retail <c>GenItemHolder::splitSize</c> and
+/// <c>GenItemHolder::maxSplitSize</c>. The toolbar entry/slider edits this state and
+/// item-holder operations consume the same value.
+/// </summary>
+public sealed class StackSplitQuantityState
+{
+    public uint Value { get; private set; } = 1u;
+    public uint Maximum { get; private set; } = 1u;
+    public float Ratio => Value / (float)Maximum;
+
+    public event Action? Changed;
+
+    public void Reset(uint stackSize, uint? initialValue = null)
+    {
+        uint maximum = Math.Max(stackSize, 1u);
+        Set(initialValue ?? maximum, maximum);
+    }
+
+    public void SetFromText(string? text)
+    {
+        // Retail uses wcstoul, whose invalid/empty result is zero, then clamps to one.
+        uint parsed = uint.TryParse(text, NumberStyles.None, CultureInfo.InvariantCulture, out uint value)
+            ? value
+            : 0u;
+        SetValue(parsed);
+    }
+
+    /// <summary>
+    /// Ports <c>gmToolbarUI::ListenToElementMessage @ 0x004BEFE0</c> and
+    /// <c>UIElement_Scrollbar::SetScrollbarPosition @ 0x00470EC0</c>. The scrollbar
+    /// first truncates its normalized position to an integer in [0,1000], then the
+    /// toolbar converts it to <c>1 + floor(position * max / 1000)</c> and clamps.
+    /// </summary>
+    public void SetFromSliderRatio(float ratio)
+    {
+        float clamped = Math.Clamp(ratio, 0f, 1f);
+        uint positionMillis = (uint)MathF.Truncate(clamped * 1000f);
+        ulong scaled = (ulong)positionMillis * Maximum;
+        uint value = 1u + (uint)(scaled / 1000u);
+        SetValue(value);
+    }
+
+    public void SetValue(uint value) => Set(value, Maximum);
+
+    private void Set(uint value, uint maximum)
+    {
+        maximum = Math.Max(maximum, 1u);
+        value = Math.Clamp(value, 1u, maximum);
+        if (Value == value && Maximum == maximum)
+            return;
+
+        Value = value;
+        Maximum = maximum;
+        Changed?.Invoke();
+    }
+}