STDOUT and STDERR

The runner is quite noisy so we're sending all `STDOUT` to `/dev/null`. However it seems like since only `printf()` is used, all output is `STDOUT` and no `STDERR` is ever sent. Could we switch from `printf()` to something like `fwrite()`?

Start by locating the runner's printf() calls and the code that redirects STDOUT to /dev/null. Trace how normal and error output are written, then verify that diagnostic output reaches STDERR while the runner remains quiet on STDOUT.

Written by the indexing model from the issue text.