refactor: extract PolygonHelper from AutoNester for shared polygon operations

Creates PolygonHelper.cs in OpenNest.Engine.BestFit with ExtractPerimeterPolygon
(returning PolygonExtractionResult with polygon + correction vector) and RotatePolygon.
AutoNester.ExtractPerimeterPolygon and RotatePolygon become thin delegates.
Adds MakeSquareDrawing/MakeLShapeDrawing to TestHelpers and 6 PolygonHelperTests.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

OpenNest.Tests/PolygonHelperTests.cs

@@ -0,0 +1,88 @@
+using OpenNest.CNC;
+using OpenNest.Engine.BestFit;
+using OpenNest.Geometry;
+using OpenNest.Math;
+
+namespace OpenNest.Tests;
+
+public class PolygonHelperTests
+{
+    [Fact]
+    public void ExtractPerimeterPolygon_ReturnsPolygon_ForValidDrawing()
+    {
+        var drawing = TestHelpers.MakeSquareDrawing();
+        var result = PolygonHelper.ExtractPerimeterPolygon(drawing, 0);
+        Assert.NotNull(result.Polygon);
+        Assert.True(result.Polygon.Vertices.Count >= 4);
+    }
+
+    [Fact]
+    public void ExtractPerimeterPolygon_InflatesPolygon_WhenSpacingNonZero()
+    {
+        var drawing = TestHelpers.MakeSquareDrawing(10);
+        var noSpacing = PolygonHelper.ExtractPerimeterPolygon(drawing, 0);
+        var withSpacing = PolygonHelper.ExtractPerimeterPolygon(drawing, 1);
+
+        noSpacing.Polygon.UpdateBounds();
+        withSpacing.Polygon.UpdateBounds();
+
+        // The offset polygon should differ in size from the non-offset polygon.
+        // OffsetSide.Left offsets outward or inward depending on winding,
+        // but either way the result must be a different size.
+        Assert.True(
+            System.Math.Abs(withSpacing.Polygon.BoundingBox.Width - noSpacing.Polygon.BoundingBox.Width) > 0.5,
+            $"Expected polygon width to differ by >0.5 with 1mm spacing. " +
+            $"No-spacing width: {noSpacing.Polygon.BoundingBox.Width:F3}, " +
+            $"With-spacing width: {withSpacing.Polygon.BoundingBox.Width:F3}");
+    }
+
+    [Fact]
+    public void ExtractPerimeterPolygon_ReturnsNull_ForEmptyDrawing()
+    {
+        var pgm = new Program();
+        var drawing = new Drawing("empty", pgm);
+        var result = PolygonHelper.ExtractPerimeterPolygon(drawing, 0);
+        Assert.Null(result.Polygon);
+    }
+
+    [Fact]
+    public void ExtractPerimeterPolygon_CorrectionVector_ReflectsOriginDifference()
+    {
+        var drawing = TestHelpers.MakeSquareDrawing();
+        var result = PolygonHelper.ExtractPerimeterPolygon(drawing, 0);
+        Assert.NotNull(result.Polygon);
+        Assert.True(System.Math.Abs(result.Correction.X) < 1);
+        Assert.True(System.Math.Abs(result.Correction.Y) < 1);
+    }
+
+    [Fact]
+    public void RotatePolygon_AtZero_ReturnsSamePolygon()
+    {
+        var polygon = new Polygon();
+        polygon.Vertices.Add(new Vector(0, 0));
+        polygon.Vertices.Add(new Vector(10, 0));
+        polygon.Vertices.Add(new Vector(10, 10));
+        polygon.Vertices.Add(new Vector(0, 10));
+        polygon.UpdateBounds();
+
+        var rotated = PolygonHelper.RotatePolygon(polygon, 0);
+        Assert.Same(polygon, rotated);
+    }
+
+    [Fact]
+    public void RotatePolygon_At90Degrees_SwapsDimensions()
+    {
+        var polygon = new Polygon();
+        polygon.Vertices.Add(new Vector(0, 0));
+        polygon.Vertices.Add(new Vector(20, 0));
+        polygon.Vertices.Add(new Vector(20, 10));
+        polygon.Vertices.Add(new Vector(0, 10));
+        polygon.UpdateBounds();
+
+        var rotated = PolygonHelper.RotatePolygon(polygon, Angle.HalfPI);
+        rotated.UpdateBounds();
+
+        Assert.True(System.Math.Abs(rotated.BoundingBox.Width - 10) < 0.1);
+        Assert.True(System.Math.Abs(rotated.BoundingBox.Length - 20) < 0.1);
+    }
+}