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>
Renamed OpenNest.Engine.Tests → OpenNest.Tests (directory, .csproj,
namespaces in all .cs files). Added OpenNest.IO project reference.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>