Remove ineffective filepath.Clean

I'm assuming that you put in this call to filepath.Clean to prevent directory traversal attacks, but it can be circumvented. You parse whatever is received with url.Parse, but url.Parse accepts relative paths. So instead of requesting gemini://gemini.lostleonardo.xyz/, one could also request something like ../certs/key.pem and titan2 would happily take that. filepath.Clean("../certs/key.pem") outputs ../certs/key.pem, so it does nothing to prevent directory traversal attacks here.

There is, however, one more thing that prevents the attack from working and that is rootDir.Open. It ensures, that no resource outside of rootDir can be accessed. Thus I think you don't even need the filepath.Clean call.