Benchmark: search on indexed mail, unique entries, QuickChart charts

benchmark.jmx
- SEARCH now filters on `mail` instead of `sn`. mail is equality-indexed by
  default on BOTH OpenDJ and OpenLDAP/osixia, whereas sn is indexed on OpenDJ
  only, so the search becomes a real indexed lookup on both servers.
- Every created value is unique: ADD keys each entry by a per-iteration JMeter
  counter, READD by a UUID. The search now matches exactly one entry, and the
  accumulated (never-deleted) READD entries no longer inflate the result set.
- Entries are minimal and index-symmetric: RDN is mail, objectClass is
  top/locality/extensibleObject, and no cn/sn/uid/givenName/telephoneNumber/
  member/uniqueMember are stored (those are indexed on OpenDJ but not osixia,
  which would bias the write cost). MODIFY writes description + userPassword.

summary.sh
- Replace the Mermaid xychart-beta charts with QuickChart (Chart.js) grouped bar
  charts rendered as images: proper side-by-side OpenLDAP/OpenDJ bars per
  operation, a legend, readable labels and no overlap. xychart-beta cannot group
  bars or show a legend and crowded the 14 x-axis labels. Throughput is a two-bar
  chart; latency is grouped bars per operation. Config is built with jq and
  URL-encoded.