Thank you so much for this answer, and also to @TerrorJack! Indeed there was something subtly wrong in my code that had nothing to do with the file system operations, but I was only able to figure it out with your suggestions and instructions for how to pipe to stdout/stderr.
As for that basic scaffolding, that’s coming direct copy/paste from ormolu live (which I think a lot of people are using as the default GHC WASM template 
). This comment about pre-initialization looks really promising, so I’m looking forward to trying that now that the basics are working.
I really appreciate the work ya’ll are putting in, it’s amazing that all of this stuff is working so well. Pretty much nothing I’ve been doing would have been possible until very recently so keep up the good work!