feat(render): Phase A8.F — ScreenPolygonClip 2D convex-polygon intersection

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/AcDream.App.Tests/Rendering/ScreenPolygonClipTests.cs

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+using System.Numerics;
+using AcDream.App.Rendering;
+using Xunit;
+
+namespace AcDream.App.Tests.Rendering;
+
+public class ScreenPolygonClipTests
+{
+    // CCW square [min,max]^2.
+    private static Vector2[] Square(float min, float max) => new[]
+    {
+        new Vector2(min, min), new Vector2(max, min), new Vector2(max, max), new Vector2(min, max),
+    };
+
+    private static float Area(Vector2[] poly)
+    {
+        if (poly.Length < 3) return 0f;
+        float a = 0f;
+        for (int i = 0; i < poly.Length; i++)
+        {
+            var p = poly[i]; var q = poly[(i + 1) % poly.Length];
+            a += p.X * q.Y - q.X * p.Y;
+        }
+        return System.MathF.Abs(a) * 0.5f;
+    }
+
+    [Fact]
+    public void Intersect_FullyContained_ReturnsSubject()
+    {
+        var outer = Square(-1f, 1f);
+        var inner = Square(-0.5f, 0.5f);
+        var r = ScreenPolygonClip.Intersect(inner, outer);
+        Assert.Equal(1.0f, Area(r), 3); // inner area = 1x1
+    }
+
+    [Fact]
+    public void Intersect_PartialOverlap_ReturnsOverlapArea()
+    {
+        var a = Square(0f, 2f);
+        var b = Square(1f, 3f);
+        var r = ScreenPolygonClip.Intersect(a, b);
+        Assert.Equal(1.0f, Area(r), 3); // overlap [1,2]^2 = 1
+    }
+
+    [Fact]
+    public void Intersect_Disjoint_ReturnsEmpty()
+    {
+        var a = Square(0f, 1f);
+        var b = Square(5f, 6f);
+        var r = ScreenPolygonClip.Intersect(a, b);
+        Assert.True(r.Length < 3); // empty
+    }
+
+    [Fact]
+    public void Intersect_Sliver_PreservesNarrowOverlap()
+    {
+        // A tall thin clip (the "stairwell") intersected with a wide subject (the "window").
+        var window = Square(-1f, 1f);
+        var stairwell = new[]
+        {
+            new Vector2(-0.1f, -1f), new Vector2(0.1f, -1f), new Vector2(0.1f, 1f), new Vector2(-0.1f, 1f),
+        };
+        var r = ScreenPolygonClip.Intersect(window, stairwell);
+        Assert.Equal(0.4f, Area(r), 3); // 0.2 wide x 2 tall
+        Assert.True(System.MathF.Abs(r[0].X) <= 0.1001f); // clipped to stairwell width
+    }
+}