pre-poweroff-load.sh runs the same mixed, deterministic workload before
every poweroff, so each observation carries identical bus exposure in both
the use_ecam=1 phase and the use_ecam=0 counter-proof. It records one line
per run (use_ecam, srcversion, blocks, warns, spd_ok) and keeps the full
per-run output beside it, then powers the machine off by itself.
Two runs of 2026-08-09 are included: without them the log is a claim.
