If the carte de legitimation
was issued by the Swiss government, please note this:
"To be granted a legitimation card, members of the staff of:
– embassies in Bern must reside in the City of Bern or within a maximum distance of 40km from the Federal capital;
– consular posts headed by a career consular representative must reside in the consular district
France would not seem to be a consular district for Geneva.
If you are indeed looking for a "idiot-proof answer", would suggest you contact an immigration attorney. This is, after all, just a general message board.