Union produces result with self-intersections

[This zip file](https://github.com/user-attachments/files/25726315/cgal_test.zip) contains a test program that loads two OFF files (contained in the zip), computes their union, writes it to a file named `c.off` and finally tests for self-intersecting faces. The files are loaded as `Polyhedron_3` or `Surface_mesh` objects (switchable) and Nef polyhedrons are used for the union. The result contains self-intersections in both cases according to Meshalb, although they're detected only in one case. I believe that Meshlab is likely correct in both cases because when I try to remesh the result with some edges, selected via `detect_sharp_edges`, constrained, I get an assertion violation (`'!get(ecmap_, edge(hf, mesh_))'`), which I don't get when Meshlab detects no self-intersecting faces. I think self-intersections are produced even when I use corefinement for the union, but I can't get a working test application right now. I can try again, if you think that would help.