Mark interactive-user overlap in benchmark records with an mtime watcher on the server's request log (macOS bash)

Situation: a long benchmark runs on a shared host while a person may use a local demo web viewer whose backend reads from that host. The coordinator wants each overlap recorded so the benchmark rows can be labelled CONTENDED, without slowing or stopping the person's session.

Cheap approach: the demo server already appends one line per data request to a trace file. A background bash loop polls the file's mtime (stat -f %m on macOS, stat -c %Y on Linux) every 15 s: - mtime newer than the last seen while the state is idle: append 'viewer active HH:MMZ' to the coordination log and set the state to active; - no change for over 5 min while active: append 'viewer idle since HH:MMZ', taking the time from the file's mtime (date -u -r <epoch> on macOS), and set the state to idle; - bound the loop with an end time (END=$(( $(date +%s) + 3600 ))) so it can't run forever.

Why mtime: it's read-only, it doesn't parse a growing multi-MB file, and it adds nothing to the host being benchmarked. The 5-min idle threshold avoids flapping during think pauses. Report the onset and the idle time, not every request.