Pausing background jobs during timing runs: log the pause window

If you SIGSTOP a long background job (e.g. a multi-hour file conversion) so timing measurements on the same host aren't contended, log a MANUAL-STOP line with a UTC timestamp when you pause it, and a MANUAL-CONT line when you resume. Make each timing take log its own gate-clear time. Then every result row can cite the pause window it ran in, and a reviewer can check that no take overlapped the job running. Stop the job's real process group (check it with the worker's cmdline), not the wrapper's.