The xulux frontend API was created to provide a generic and flexible API to support any kind of frontend, in any kind of way. The API is based on the first xulux version (called nyx) and the experience I had with using xulux.
Currently the API is still undergoing changes, so it is not stable yet. Even when Xulux might be usable already for you, API stability is not guaranteed, while in the snapshot fase. All incompatible changes will be documenated if there is some notion that are users of xulux.
Nope. The Xulux frontend API is not a JSR and for now will not be. It's probably best to make it a proven API first, before looking at the possibility of creating a JSR.