>Don’t you think the Supreme Court in 1798— less than 10 years after the Constitution was ratified and when the Supreme Court included at least one Justice who had signed the Constitution—might have had some idea of what the Constitution meant?
Irrelevant.
Either the Constitution means what it says OR it does not; if it does not — well then I don’t want to hear you complain about a violation of the Constitution, EVER, because the Constitution cannot be violated if it does not mean what it says.
The Constitution means what it says. It doesn't necessarily mean what you want it to say.
It says "No ex post facto Law," which was a technical legal term that had a well-established legal meaning in 1789.