Efficient group by on sorted CSV using PostgreSQL FDW
I'm afraid not.

There is no such thing as a "sorted csv" file format, all you can have is a csv file that just happens to be sorted. I could take your example file and move the top line to the bottom and it would still be a valid csv.

If the fdw made that trusted your assertion and you made a mistake it would return incorrect results which is obviously unacceptable.

So, you will either want to import that file if you are doing lots of processing, convert it to a sorted db file format that fdw supports or pre summarize the data if that is all you are doing.