In the self.last_sec > o.last_sec branch, merge forwarded a clone of o to
age out stale ring-buffer slots but then summed the un-forwarded o.totals
instead of the forwarded copy, so aged-out windows leaked into the result
and the forwarded clone was a dead write. Read both operands in forwarded
form and align the summation on wrapping_add to match AccElem::merge.
