fix: resolve rendering issues when applying lead-ins to parts

Three issues caused incorrect rendering after lead-in application:
- Rapid move entities from ToGeometry() were included in ShapeProfile
  contour detection, turning traversal paths into cutting moves
- Program created with Mode.Incremental default made the absolute-to-
  incremental conversion a no-op, leaving coordinates unconverted
- AddProgramSplit didn't call StartFigure() at rapid moves, causing
  GraphicsPath to draw implicit connecting lines between contours
- Part.Rotation returned 0 from the new program instead of the actual
  rotation, displacing the sequence label on rotated parts

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

OpenNest.Core/Part.cs

@@ -94,7 +94,7 @@ namespace OpenNest
         /// </summary>
         public double Rotation
         {
-            get { return Program.Rotation; }
+            get { return HasManualLeadIns ? preLeadInRotation : Program.Rotation; }
         }
 
         /// <summary>