Diagnostic-only counters updated at the end of every sweep() — for the
periodic mem_attribution log (sysinfo.rs) to correlate commit/handle
growth against reaper activity. A reaper_candidates/reaper_first_seen
that climbs sweep over sweep without a matching rise in
reaper_reaped_cumulative would mean the reaper keeps finding (but not
managing to close) the same growing set of blocks — worth seeing
directly rather than inferred from commit numbers alone.
One sweep. first_seen carries each candidate’s first sighting across
sweeps; a block that got placed (or deleted) in between drops out of it.
Returns the block ids it closed.