feat(core): add TrapezoidShape

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

OpenNest.Core/Shapes/TrapezoidShape.cs

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+using System.Collections.Generic;
+using OpenNest.Geometry;
+
+namespace OpenNest.Shapes
+{
+    public class TrapezoidShape : ShapeDefinition
+    {
+        public double TopWidth { get; set; }
+        public double BottomWidth { get; set; }
+        public double Height { get; set; }
+
+        public override Drawing GetDrawing()
+        {
+            var offset = (BottomWidth - TopWidth) / 2.0;
+
+            var entities = new List<Entity>
+            {
+                new Line(0, 0, BottomWidth, 0),
+                new Line(BottomWidth, 0, offset + TopWidth, Height),
+                new Line(offset + TopWidth, Height, offset, Height),
+                new Line(offset, Height, 0, 0)
+            };
+
+            return CreateDrawing(entities);
+        }
+    }
+}

OpenNest.Tests/Shapes/TrapezoidShapeTests.cs

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+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);
+    }
+}