feat(D.5.1): UiItemSlot widget (UIElement_UIItem cell port)

Behavioral leaf widget for the toolbar item cell. Draws the empty-slot
sprite (0x060074CF) when unbound; draws the pre-composited icon texture
when a weenie is bound via SetItem(). ConsumesDatChildren=true prevents
the LayoutImporter from double-building the dat sub-elements. SpriteResolve
is configurable so paperdoll equip slots can swap in per-slot silhouettes
later. No Clicked/OnEvent — that wiring comes in Task 8 (ToolbarController).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/AcDream.App/UI/UiItemSlot.cs

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+using System;
+using System.Numerics;
+
+namespace AcDream.App.UI;
+
+/// <summary>
+/// One item-in-a-slot cell (port of retail UIElement_UIItem, class 0x10000032).
+/// A behavioral LEAF: it draws the empty-slot sprite when unbound, else a
+/// pre-composited icon texture (set by the controller). Holds the bound weenie
+/// guid (retail UIElement_UIItem::itemID, +0x5FC).
+/// </summary>
+public sealed class UiItemSlot : UiElement
+{
+    public UiItemSlot() { ClickThrough = false; }
+
+    public override bool ConsumesDatChildren => true;
+
+    /// <summary>Bound weenie guid (0 = empty). Retail UIElement_UIItem::itemID.</summary>
+    public uint ItemId { get; private set; }
+
+    /// <summary>Pre-composited icon GL texture for the bound item (0 = none).</summary>
+    public uint IconTexture { get; private set; }
+
+    /// <summary>Empty-slot sprite. Default = the generic toolbar empty-slot border
+    /// 0x060074CF (uiitem template 0x21000037, state ItemSlot_Empty). Configurable so
+    /// paperdoll equip slots can use their per-slot silhouettes later.</summary>
+    public uint EmptySprite { get; set; } = 0x060074CFu;
+
+    /// <summary>RenderSurface id -> (GL texture, w, h). Set by the factory/controller.</summary>
+    public Func<uint, (uint tex, int w, int h)>? SpriteResolve { get; set; }
+
+    public void SetItem(uint itemId, uint iconTexture)
+    {
+        ItemId = itemId;
+        IconTexture = iconTexture;
+    }
+
+    public void Clear() { ItemId = 0; IconTexture = 0; }
+
+    protected override void OnDraw(UiRenderContext ctx)
+    {
+        if (ItemId != 0 && IconTexture != 0)
+        {
+            ctx.DrawSprite(IconTexture, 0f, 0f, Width, Height, 0f, 0f, 1f, 1f, Vector4.One);
+            return;
+        }
+        if (SpriteResolve is not null && EmptySprite != 0)
+        {
+            var (tex, _, _) = SpriteResolve(EmptySprite);
+            if (tex != 0)
+                ctx.DrawSprite(tex, 0f, 0f, Width, Height, 0f, 0f, 1f, 1f, Vector4.One);
+        }
+    }
+}

tests/AcDream.App.Tests/UI/UiItemSlotTests.cs

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+using AcDream.App.UI;
+
+namespace AcDream.App.Tests.UI;
+
+public class UiItemSlotTests
+{
+    [Fact]
+    public void IsLeafWidget()
+        => Assert.True(new UiItemSlot().ConsumesDatChildren);
+
+    [Fact]
+    public void DefaultEmptySprite_isToolbarBorder()
+        => Assert.Equal(0x060074CFu, new UiItemSlot().EmptySprite);
+
+    [Fact]
+    public void Empty_whenNoItem()
+    {
+        var s = new UiItemSlot();
+        Assert.Equal(0u, s.ItemId);
+        Assert.Equal(0u, s.IconTexture);
+    }
+
+    [Fact]
+    public void SetItem_setsIdAndTexture()
+    {
+        var s = new UiItemSlot();
+        s.SetItem(0x5001u, 0x99u);
+        Assert.Equal(0x5001u, s.ItemId);
+        Assert.Equal(0x99u, s.IconTexture);
+    }
+}