Hey, thanks again for letting me know, I figured out the problem. Turns out I remembered to set up a keystore, but I forgot to actually _enable_ package signing. That was a silly thing to do and Android was right to complain about it (with an "INSTALL_PARSE_FAILED_NO_CERTIFICATES" error.)
I've uploaded a fixed version and confirmed that I can download and install it on my phone. So, it should work now.