Fix benchmark charts: non-overlapping columns and total throughput

Two report fixes in summary.sh.

- Make the per-operation columns actually render side by side. xychart-beta
  merges duplicate x-axis category labels into one slot, so the previous
  "ADD","ADD" axis collapsed both series back onto a single column and the
  OpenLDAP/OpenDJ bars overlapped. Use distinct labels ("ADD OL","ADD DJ", ...)
  so the zero-padded series stay on separate columns.
- Replace per-operation throughput with a single total-throughput comparison.
  In a sequential loop every operation runs once per iteration, so per-op
  throughput just equals the loop rate (nearly identical across ops, which
  looked wrong). The per-operation table now shows latency only (mean/p99/
  errors); a two-bar chart shows total OpenLDAP vs OpenDJ throughput, with a
  note explaining why per-op throughput is not charted.

Verified by rendering the report from the real run's statistics.json artifacts.