feat(render): Phase A8.F — ViewPolygon + CellView clip-region data model

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

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

@@ -0,0 +1,52 @@
+using System.Numerics;
+using AcDream.App.Rendering;
+using Xunit;
+
+namespace AcDream.App.Tests.Rendering;
+
+public class PortalViewTests
+{
+    [Fact]
+    public void ViewPolygon_ComputesBoundingRect()
+    {
+        var p = new ViewPolygon(new[]
+        {
+            new Vector2(-0.5f, -0.25f), new Vector2(0.5f, -0.25f), new Vector2(0.0f, 0.75f),
+        });
+        Assert.Equal(-0.5f, p.MinX, 5);
+        Assert.Equal(0.5f, p.MaxX, 5);
+        Assert.Equal(-0.25f, p.MinY, 5);
+        Assert.Equal(0.75f, p.MaxY, 5);
+        Assert.False(p.IsEmpty);
+    }
+
+    [Fact]
+    public void ViewPolygon_FewerThanThreeVerts_IsEmpty()
+    {
+        Assert.True(new ViewPolygon(new[] { new Vector2(0, 0), new Vector2(1, 0) }).IsEmpty);
+        Assert.True(new ViewPolygon(System.Array.Empty<Vector2>()).IsEmpty);
+    }
+
+    [Fact]
+    public void CellView_FullScreen_CoversNdc()
+    {
+        var v = CellView.FullScreen();
+        Assert.False(v.IsEmpty);
+        Assert.Equal(-1f, v.MinX, 5);
+        Assert.Equal(1f, v.MaxX, 5);
+        Assert.Equal(-1f, v.MinY, 5);
+        Assert.Equal(1f, v.MaxY, 5);
+    }
+
+    [Fact]
+    public void CellView_Add_GrowsUnionBoundsAndIsEmptyTracks()
+    {
+        var v = new CellView();
+        Assert.True(v.IsEmpty);
+        v.Add(new ViewPolygon(new[] { new Vector2(0, 0), new Vector2(0.2f, 0), new Vector2(0, 0.2f) }));
+        v.Add(new ViewPolygon(new[] { new Vector2(-0.3f, -0.3f), new Vector2(-0.1f, -0.3f), new Vector2(-0.1f, -0.1f) }));
+        Assert.False(v.IsEmpty);
+        Assert.Equal(-0.3f, v.MinX, 5);
+        Assert.Equal(0.2f, v.MaxX, 5);
+    }
+}