The ambient endpoint writes calendar_event_* and never calendar_busy, so a
notification-derived meeting was good enough to recite out loud and not good
enough to stop a nudge during it. Backwards: being wrong here costs one nudge.
The loop gatherer now derives busy from the event facts themselves, so the
expiry IS the meeting's span. No new level, no interval to choose, and no way
for the suppression to outlive the meeting. calendar.FactSpan reads back what
FactValue wrote; anything that does not parse says nothing about now.
