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.App/UI/UiScrollbar.cs

@@ -25,6 +25,14 @@ public sealed class UiScrollbar : UiElement
     /// <summary>The scroll model this bar reflects + drives (shared with the transcript).</summary>
     public UiScrollable? Model { get; set; }
 
+    /// <summary>
+    /// Optional scalar mode used by retail's horizontal stack-split control. When set,
+    /// the bar reflects/drives one normalized value rather than a <see cref="UiScrollable"/>.
+    /// </summary>
+    public Func<float>? ScalarValue { get; set; }
+    public Action<float>? ScalarChanged { get; set; }
+    public bool Horizontal { get; set; }
+
     /// <summary>RenderSurface id → (GL tex, w, h). 0 id = skip.</summary>
     public Func<uint, (uint tex, int w, int h)>? SpriteResolve { get; set; }
 
@@ -58,6 +66,7 @@ public sealed class UiScrollbar : UiElement
 
     private bool _draggingThumb;
     private float _dragOffsetY;
+    private float _dragOffsetX;
 
     public UiScrollbar() { CapturesPointerDrag = true; }
 
@@ -85,7 +94,18 @@ public sealed class UiScrollbar : UiElement
 
     protected override void OnDraw(UiRenderContext ctx)
     {
-        if (Model is not { } m || SpriteResolve is not { } resolve) return;
+        if (SpriteResolve is not { } resolve) return;
+        if (Horizontal && ScalarValue is not null)
+        {
+            DrawTiled(ctx, resolve, TrackSprite, 0f, 0f, Width, Height);
+            float thumbWidth = ScalarThumbWidth(resolve);
+            float travel = MathF.Max(0f, Width - thumbWidth);
+            float x = travel * Math.Clamp(ScalarValue(), 0f, 1f);
+            DrawSprite(ctx, resolve, ThumbSprite, x, 0f, thumbWidth, Height);
+            return;
+        }
+
+        if (Model is not { } m) return;
 
         // Track background — TILED vertically (retail DrawMode=Normal). The native track
         // sprite (~16×32) repeats to fill the element height instead of stretch-distorting.
@@ -142,6 +162,9 @@ public sealed class UiScrollbar : UiElement
 
     public override bool OnEvent(in UiEvent e)
     {
+        if (Horizontal && ScalarValue is not null && ScalarChanged is not null)
+            return OnScalarEvent(e);
+
         if (Model is not { } m) return false;
 
         switch (e.Type)
@@ -196,4 +219,53 @@ public sealed class UiScrollbar : UiElement
 
         return false;
     }
+
+    private bool OnScalarEvent(in UiEvent e)
+    {
+        switch (e.Type)
+        {
+            case UiEventType.MouseDown:
+            {
+                float thumbWidth = ScalarThumbWidth(SpriteResolve);
+                float travel = MathF.Max(1f, Width - thumbWidth);
+                float thumbX = travel * Math.Clamp(ScalarValue!(), 0f, 1f);
+                float x = e.Data1;
+                if (x >= thumbX && x <= thumbX + thumbWidth)
+                {
+                    _dragOffsetX = x - thumbX;
+                }
+                else
+                {
+                    _dragOffsetX = thumbWidth * 0.5f;
+                    ScalarChanged!(Math.Clamp((x - _dragOffsetX) / travel, 0f, 1f));
+                }
+                _draggingThumb = true;
+                return true;
+            }
+
+            case UiEventType.MouseMove when _draggingThumb:
+            {
+                float thumbWidth = ScalarThumbWidth(SpriteResolve);
+                float travel = MathF.Max(1f, Width - thumbWidth);
+                ScalarChanged!(Math.Clamp(((float)e.Data1 - _dragOffsetX) / travel, 0f, 1f));
+                return true;
+            }
+
+            case UiEventType.MouseUp:
+                _draggingThumb = false;
+                return true;
+        }
+
+        return false;
+    }
+
+    private float ScalarThumbWidth(Func<uint, (uint tex, int w, int h)>? resolve)
+    {
+        if (resolve is not null && ThumbSprite != 0)
+        {
+            var (_, width, _) = resolve(ThumbSprite);
+            if (width > 0) return MathF.Min(width, Width);
+        }
+        return MathF.Min(16f, Width);
+    }
 }