fix(core): keep lead-in state and rotation through part copy and rotate

Part.Clone and CloneAtOffset copied the lead-in program but not
HasManualLeadIns, LeadInsLocked, CuttingParameters or the tracked
pre-lead-in rotation. A copy of a rotated lead-in part reported
rotation 0 (the rebuilt program's), so saving it wrote the wrong
rotation and Remove Lead-ins restored an unrotated part.

Part.Rotate on a lead-in part set the tracked rotation from the
lead-in program's own Rotation, which starts at zero when the cutting
strategy rebuilds it, so a further quarter turn left Rotation unchanged.
Lead-in parts now accumulate the applied angle instead.

OpenNest.Core/Part.cs

@@ -145,7 +145,7 @@ namespace OpenNest
             EnsureOwnedProgram();
             Program.Rotate(angle);
             location = Location.Rotate(angle);
-            preLeadInRotation = Program.Rotation;
+            TrackRotation(angle);
             UpdateBounds();
         }
 
@@ -159,10 +159,22 @@ namespace OpenNest
             EnsureOwnedProgram();
             Program.Rotate(angle);
             location = Location.Rotate(angle, origin);
-            preLeadInRotation = Program.Rotation;
+            TrackRotation(angle);
             UpdateBounds();
         }
 
+        /// <summary>
+        /// Records the part's rotation after it turned by <paramref name="angle"/>. A lead-in
+        /// program is rebuilt by the cutting strategy and starts over at zero program
+        /// rotation, so for those parts the rotation is accumulated rather than read back.
+        /// </summary>
+        private void TrackRotation(double angle)
+        {
+            preLeadInRotation = HasManualLeadIns
+                ? Angle.NormalizeRad(preLeadInRotation + angle)
+                : Program.Rotation;
+        }
+
         /// <summary>
         /// Offsets the part.
         /// </summary>
@@ -323,6 +335,7 @@ namespace OpenNest
                 new Box(BoundingBox.X, BoundingBox.Y, BoundingBox.Length, BoundingBox.Width)
             );
             part.ownsProgram = true;
+            part.CopyLeadInStateFrom(this);
 
             return part;
         }
@@ -347,10 +360,24 @@ namespace OpenNest
                     BoundingBox.Width
                 )
             );
+            part.CopyLeadInStateFrom(this);
 
             return part;
         }
 
+        /// <summary>
+        /// Copies the lead-in state that goes with a copied program. Without it a copy of a
+        /// lead-in part reads its rotation from the rebuilt program (zero), and lead-in
+        /// assignment does not know to strip the copied lead-ins before adding new ones.
+        /// </summary>
+        private void CopyLeadInStateFrom(Part source)
+        {
+            HasManualLeadIns = source.HasManualLeadIns;
+            LeadInsLocked = source.LeadInsLocked;
+            CuttingParameters = source.CuttingParameters;
+            preLeadInRotation = source.preLeadInRotation;
+        }
+
         private void EnsureOwnedProgram()
         {
             if (!ownsProgram)