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.Engine/BestFit/PolygonHelper.cs

@@ -0,0 +1,89 @@
+using OpenNest.Converters;
+using OpenNest.Geometry;
+using OpenNest.Math;
+using System.Linq;
+
+namespace OpenNest.Engine.BestFit
+{
+    public static class PolygonHelper
+    {
+        public static PolygonExtractionResult ExtractPerimeterPolygon(Drawing drawing, double halfSpacing)
+        {
+            var entities = ConvertProgram.ToGeometry(drawing.Program)
+                .Where(e => e.Layer != SpecialLayers.Rapid)
+                .ToList();
+
+            if (entities.Count == 0)
+                return new PolygonExtractionResult(null, Vector.Zero);
+
+            var definedShape = new ShapeProfile(entities);
+            var perimeter = definedShape.Perimeter;
+
+            if (perimeter == null)
+                return new PolygonExtractionResult(null, Vector.Zero);
+
+            // Compute the perimeter bounding box before inflation for coordinate correction.
+            perimeter.UpdateBounds();
+            var perimeterBb = perimeter.BoundingBox;
+
+            // Inflate by half-spacing if spacing is non-zero.
+            Shape inflated;
+
+            if (halfSpacing > 0)
+            {
+                var offsetEntity = perimeter.OffsetEntity(halfSpacing, OffsetSide.Left);
+                inflated = offsetEntity as Shape ?? perimeter;
+            }
+            else
+            {
+                inflated = perimeter;
+            }
+
+            // Convert to polygon with circumscribed arcs for tight nesting.
+            var polygon = inflated.ToPolygonWithTolerance(0.01, circumscribe: true);
+
+            if (polygon.Vertices.Count < 3)
+                return new PolygonExtractionResult(null, Vector.Zero);
+
+            // Compute correction: difference between program origin and perimeter origin.
+            // Part.CreateAtOrigin normalizes to program bbox; polygon normalizes to perimeter bbox.
+            var programBb = drawing.Program.BoundingBox();
+            var correction = new Vector(
+                perimeterBb.Left - programBb.Location.X,
+                perimeterBb.Bottom - programBb.Location.Y);
+
+            // Normalize: move reference point to origin.
+            polygon.UpdateBounds();
+            var bb = polygon.BoundingBox;
+            polygon.Offset(-bb.Left, -bb.Bottom);
+
+            return new PolygonExtractionResult(polygon, correction);
+        }
+
+        public static Polygon RotatePolygon(Polygon polygon, double angle)
+        {
+            if (angle.IsEqualTo(0))
+                return polygon;
+
+            var result = new Polygon();
+            var cos = System.Math.Cos(angle);
+            var sin = System.Math.Sin(angle);
+
+            foreach (var v in polygon.Vertices)
+            {
+                result.Vertices.Add(new Vector(
+                    v.X * cos - v.Y * sin,
+                    v.X * sin + v.Y * cos));
+            }
+
+            // Re-normalize to origin.
+            result.UpdateBounds();
+            var bb = result.BoundingBox;
+            result.Offset(-bb.Left, -bb.Bottom);
+
+            return result;
+        }
+    }
+
+    public record PolygonExtractionResult(Polygon Polygon, Vector Correction);
+}