fix: preserve part rotation through lead-in assign/remove cycle

Track preLeadInRotation when parts are rotated so lead-in removal
can restore the correct rotation. Remove stale HasManualLeadIns and
LeadInsLocked deserialization from NestReader since these flags are
transient state, not persisted data.

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

OpenNest.Core/Part.cs

@@ -106,6 +106,7 @@ namespace OpenNest
             EnsureOwnedProgram();
             Program.Rotate(angle);
             location = Location.Rotate(angle);
+            preLeadInRotation = Program.Rotation;
             UpdateBounds();
         }
 
@@ -119,6 +120,7 @@ namespace OpenNest
             EnsureOwnedProgram();
             Program.Rotate(angle);
             location = Location.Rotate(angle, origin);
+            preLeadInRotation = Program.Rotation;
             UpdateBounds();
         }