Negative & large shifts are implementation specific. This should be made clear in the specification, and some testing should be done on various implementations/hosts to document the range of behaviors.
This should probably check ./library then /usr/local/share/retro12/library when loading.
RetroForth 11 had support for loading files from a local library (from a "library" directory in the current directory). It might be useful to have something similar to this in RetroForth/nga.