feat(D.2b): Slice 2 — UiViewport widget (dat Type 0xD) + IUiViewportRenderer seam

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

src/AcDream.App/UI/UiViewport.cs

@@ -0,0 +1,25 @@
+using System.Numerics;
+
+namespace AcDream.App.UI;
+
+/// <summary>Leaf widget for dat Type 0xD (UIElement_Viewport). Blits the texture produced by its
+/// IUiViewportRenderer (run in the pre-UI hook) as a single sprite at its own rect. The 3-D render
+/// does NOT happen here (OnDraw only has a 2-D context).</summary>
+public sealed class UiViewport : UiElement
+{
+    public override bool ConsumesDatChildren => true;
+
+    /// <summary>Renderer that produces the off-screen texture. Set by GameWindow wiring (later task).</summary>
+    public IUiViewportRenderer? Renderer { get; set; }
+
+    /// <summary>Last GL color-texture handle produced by the pre-UI hook. 0 = nothing to blit.</summary>
+    public uint TextureHandle { get; set; }
+
+    protected override void OnDraw(UiRenderContext ctx)
+    {
+        if (!Visible || TextureHandle == 0) return;
+        // Local origin is already at this widget's Left/Top (PushTransform applied by DrawSelfAndChildren).
+        // Draw the full-texture sprite with UV 0..1 and white tint (no color modulation).
+        ctx.DrawSprite(TextureHandle, 0f, 0f, Width, Height, 0f, 0f, 1f, 1f, Vector4.One);
+    }
+}