feat(D.2b): UiNineSlicePanel — 8-piece retail window frame + geometry test

Implements the retail floating-window bevel as a UiPanel subclass using
RetailChromeSprites: 4 tiled edges + 4 stretched corners + tiled center fill,
matching the 8-piece border layout confirmed by the D.2b Step-0 prove-out.
Resolver delegate keeps GL out of unit tests. Geometry verified by
ComputeFrameRects_PlacesCornersEdgesAndCenter (1/1 pass).

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

src/AcDream.App/UI/UiNineSlicePanel.cs

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+using System.Numerics;
+
+namespace AcDream.App.UI;
+
+/// <summary>
+/// A <see cref="UiPanel"/> whose background is the retail 8-piece window bevel
+/// (<see cref="RetailChromeSprites"/>): 4 corners + 4 edges around a tiled
+/// center fill. Retires the flat translucent rect (divergence row TS-30).
+/// Sprites resolve to (GL handle, width, height) via an injected delegate so
+/// the widget is testable without GL. In production:
+/// <c>id => { var t = cache.GetOrUploadRenderSurface(id, out var w, out var h); return (t, w, h); }</c>.
+/// </summary>
+public sealed class UiNineSlicePanel : UiPanel
+{
+    /// <summary>A placed chrome piece: destination rect in local pixel space.</summary>
+    public readonly record struct Rect(float X, float Y, float W, float H);
+
+    /// <summary>The nine destination rects for an 8-piece border + center.</summary>
+    public readonly record struct FrameRects(
+        Rect Center, Rect Top, Rect Bottom, Rect Left, Rect Right,
+        Rect TL, Rect TR, Rect BL, Rect BR);
+
+    private readonly System.Func<uint, (uint tex, int w, int h)> _resolve;
+
+    public UiNineSlicePanel(System.Func<uint, (uint, int, int)> resolve)
+    {
+        _resolve = resolve;
+        BackgroundColor = Vector4.Zero; // suppress the base flat-rect fill
+        BorderColor = Vector4.Zero;
+    }
+
+    /// <summary>
+    /// Destination rects (local px) for a frame of (<paramref name="w"/>,
+    /// <paramref name="h"/>) with border thickness <paramref name="b"/>:
+    /// b×b corners, top/bottom edges spanning the interior width at height b,
+    /// left/right edges spanning the interior height at width b, center fills
+    /// the interior.
+    /// </summary>
+    public static FrameRects ComputeFrameRects(float w, float h, int b)
+    {
+        float innerW = w - 2 * b;
+        float innerH = h - 2 * b;
+        return new FrameRects(
+            Center: new Rect(b, b, innerW, innerH),
+            Top:    new Rect(b, 0, innerW, b),
+            Bottom: new Rect(b, h - b, innerW, b),
+            Left:   new Rect(0, b, b, innerH),
+            Right:  new Rect(w - b, b, b, innerH),
+            TL: new Rect(0, 0, b, b),
+            TR: new Rect(w - b, 0, b, b),
+            BL: new Rect(0, h - b, b, b),
+            BR: new Rect(w - b, h - b, b, b));
+    }
+
+    protected override void OnDraw(UiRenderContext ctx)
+    {
+        var r = ComputeFrameRects(Width, Height, RetailChromeSprites.Border);
+        // center + edges tile (UV repeat); corners stretch 1:1.
+        DrawTiled(ctx, RetailChromeSprites.CenterFill, r.Center);
+        DrawTiled(ctx, RetailChromeSprites.TopEdge,    r.Top);
+        DrawTiled(ctx, RetailChromeSprites.BottomEdge, r.Bottom);
+        DrawTiled(ctx, RetailChromeSprites.LeftEdge,   r.Left);
+        DrawTiled(ctx, RetailChromeSprites.RightEdge,  r.Right);
+        DrawStretched(ctx, RetailChromeSprites.CornerTL, r.TL);
+        DrawStretched(ctx, RetailChromeSprites.CornerTR, r.TR);
+        DrawStretched(ctx, RetailChromeSprites.CornerBL, r.BL);
+        DrawStretched(ctx, RetailChromeSprites.CornerBR, r.BR);
+    }
+
+    private void DrawTiled(UiRenderContext ctx, uint id, Rect d)
+    {
+        if (d.W <= 0 || d.H <= 0) return;
+        var (tex, tw, th) = _resolve(id);
+        if (tex == 0 || tw == 0 || th == 0) return;
+        ctx.DrawSprite(tex, d.X, d.Y, d.W, d.H, 0, 0, d.W / tw, d.H / th, Vector4.One);
+    }
+
+    private void DrawStretched(UiRenderContext ctx, uint id, Rect d)
+    {
+        if (d.W <= 0 || d.H <= 0) return;
+        var (tex, _, _) = _resolve(id);
+        if (tex == 0) return;
+        ctx.DrawSprite(tex, d.X, d.Y, d.W, d.H, 0, 0, 1, 1, Vector4.One);
+    }
+}

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

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+using AcDream.App.UI;
+
+namespace AcDream.App.Tests.UI;
+
+public class UiNineSlicePanelTests
+{
+    [Fact]
+    public void ComputeFrameRects_PlacesCornersEdgesAndCenter()
+    {
+        var r = UiNineSlicePanel.ComputeFrameRects(100, 80, 5);
+
+        // 5x5 corners at the four corners
+        Assert.Equal(new UiNineSlicePanel.Rect(0,  0,  5, 5), r.TL);
+        Assert.Equal(new UiNineSlicePanel.Rect(95, 0,  5, 5), r.TR);
+        Assert.Equal(new UiNineSlicePanel.Rect(0,  75, 5, 5), r.BL);
+        Assert.Equal(new UiNineSlicePanel.Rect(95, 75, 5, 5), r.BR);
+
+        // edges span the interior (100-2*5 = 90 wide, 80-2*5 = 70 tall)
+        Assert.Equal(new UiNineSlicePanel.Rect(5,  0,  90, 5),  r.Top);
+        Assert.Equal(new UiNineSlicePanel.Rect(5,  75, 90, 5),  r.Bottom);
+        Assert.Equal(new UiNineSlicePanel.Rect(0,  5,  5,  70), r.Left);
+        Assert.Equal(new UiNineSlicePanel.Rect(95, 5,  5,  70), r.Right);
+
+        // center fills the interior
+        Assert.Equal(new UiNineSlicePanel.Rect(5, 5, 90, 70), r.Center);
+    }
+}