The only sensible normalisation one can implement is the full decomposition (NFD), and maybe the full composition (NFC). You rely on the full decomposition if you want to collate correctly, which is a problem because the amount of memory needed to store the decomposition is unbounded in general. I don't want to make the libgrapheme users jump through hoops, and I also don't want to do any memory allocations in libgrapheme either.
There is an idea floating in my head on how to solve this, but I'm currently busy finalising Unicode 15.1 support (Unicode 16.0, released on the 10th, will be trivial to upgrade to) and releasing my already fully-compliant implementation of the Unicode bidirectional algorithm.