Checkpoint
One last thing before we move on. Same surface as a write step — but the lesson doesn't complete until this passes.
Build the doc validator. The system last changed on day 385. A section is MISSING when not done; a done section is STALE when its updated_on_day predates the system's last change. Print f"missing: {list}", f"stale (older than the system's last change): {list}", and the verdict: "trustworthy" only when both lists are empty, else "misleads with authority - fix before sharing". A stale architecture doc is the exact fate the audit discipline exists to prevent.