chore: add shared test helpers for Engine tests

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

OpenNest.Engine.Tests/TestHelpers.cs

@@ -0,0 +1,27 @@
+using OpenNest.CNC;
+using OpenNest.Geometry;
+
+namespace OpenNest.Engine.Tests;
+
+internal static class TestHelpers
+{
+    public static Part MakePartAt(double x, double y, double size = 1)
+    {
+        var pgm = new Program();
+        pgm.Codes.Add(new RapidMove(new Vector(0, 0)));
+        pgm.Codes.Add(new LinearMove(new Vector(size, 0)));
+        pgm.Codes.Add(new LinearMove(new Vector(size, size)));
+        pgm.Codes.Add(new LinearMove(new Vector(0, size)));
+        pgm.Codes.Add(new LinearMove(new Vector(0, 0)));
+        var drawing = new Drawing("test", pgm);
+        return new Part(drawing, new Vector(x, y));
+    }
+
+    public static Plate MakePlate(double width = 60, double length = 120, params Part[] parts)
+    {
+        var plate = new Plate(width, length);
+        foreach (var p in parts)
+            plate.Parts.Add(p);
+        return plate;
+    }
+}