feat(core): add TrapezoidShape

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.Tests/Shapes/TrapezoidShapeTests.cs

@@ -0,0 +1,27 @@
+using OpenNest.Shapes;
+
+namespace OpenNest.Tests.Shapes;
+
+public class TrapezoidShapeTests
+{
+    [Fact]
+    public void GetDrawing_BoundingBoxMatchesDimensions()
+    {
+        var shape = new TrapezoidShape { BottomWidth = 20, TopWidth = 10, Height = 8 };
+        var drawing = shape.GetDrawing();
+
+        var bbox = drawing.Program.BoundingBox();
+        Assert.Equal(20, bbox.Width, 0.01);
+        Assert.Equal(8, bbox.Length, 0.01);
+    }
+
+    [Fact]
+    public void GetDrawing_AreaIsCorrect()
+    {
+        var shape = new TrapezoidShape { BottomWidth = 20, TopWidth = 10, Height = 8 };
+        var drawing = shape.GetDrawing();
+
+        // Area = (top + bottom) / 2 * height = (10 + 20) / 2 * 8 = 120
+        Assert.Equal(120, drawing.Area, 0.5);
+    }
+}