FR: 0_append for csvs

Excuse my general C naivete but would it be possible at all to have stan write it's csvs using O_APPEND mode whereby thw writer looks for the end of the file before every write? While waiting for the proper IO refactor I've been coding a csv->Inference Data converter that watches the csvs as they're created, grabs any new content then runs truncate -s 0 on the file to save on disk space. I can obviously skip the truncation part if it turns out that the csv writer here can't do O_APPEND or if there's a non-negligible implementation or performance cost, but thought I'd check.

The issue does not name a file, test, or entry point. Locate the CSV writer and first inspect how it opens and writes output files, then assess whether O_APPEND fits the current IO design and performance constraints. Done means the feasibility and expected behavior are established, with relevant coverage if a change is warranted.