OK, so having spent far too damned long trying to understand SNCF's ridiculous luggage rules, I have now written a guide - to help you avoid arguments and a possible €50 fine, and respecting your own safety and privacy as far as you can, within the rules

Thanks in particular to @bohwaz for the help!

https://jonworth.eu/how-to-not-fall-foul-of-sncfs-luggage-rules/