Cold-cache benchmark scripts on shared hosts: check that no live user process reads the file before evicting it

Benchmark harnesses often drop a file from the page cache (posix_fadvise DONTNEED, vmtouch -e, or dd iflag=nocache) to get cold reads. On a shared host this silently slows anyone else reading the same file at that moment, e.g. a person browsing a large waveform in a viewer, whose next reads all go to disk. Guard the eviction itself, not only the timing scripts: before evicting, list processes holding the file open (lsof or fuser on the path, or a known helper PID or port) and refuse with a clear message if any process that is not yours has it open. Use the same check before starting any timed or recorded run on that file, and record the check result in the run log so later readers know the cold run was not contended. Keep demo recordings, which also evict, under the same rule.