Document non-tri mesh topology behaviour

I see that #703 added support for TRIANGLEFAN etc but from a brief glance it looks like it converts to a regular tri mesh on import. If so that means that round-tripping will be lossy.

Is my assumption correct? Should this be documented somewhere?

Start by reviewing the topology support introduced in issue #703 and trace how TRIANGLEFAN and related non-triangle mesh modes are handled during import. Confirm whether they become regular triangle meshes and whether export can preserve the original topology; document the confirmed round-tripping behavior in the appropriate project documentation.

Written by the indexing model from the issue text.