I would like to know if I can buy some gizmo that can provide a
definite, positive stop to the lathe carriage before the cutter or the
carriage hits the chuck. Any suggestions, at least what are these
things called.

You got the name right. They also make micrometer carriage stops
so that you can move the carriage a controlled amount.

I often use a carriage stop when turning to a shoulder. After the
bulk of the turning is done, I can adjust the micrometer dial a
little to take a cleanup cut on the shoulder or get the final
dimension correct.

My atlas lathe came with a homemade carraige stop. Since the atlas
was a flat bed, it was very easy to make something that clamped
to the bed with a fine thread screw sticking out the side that
stopped the carriage.

My rockwell lathe has a V way so the carriage stop has to have a
matching V. Same principle, but this one also has a micrometer head
for precise adjustment.

chuck