Refine LDAP benchmark: READD, SSHA-256 hashing, grouped charts

Five follow-up tweaks to the benchmark workflow and plan.

- Rename the "ADD WITHOUT DELETE" sampler to READD (benchmark.jmx, summary.sh OPS).
- Stop each Docker container right after its own benchmark (new Stop OpenLDAP /
  Stop OpenDJ steps) to free resources and keep only one server under load at a
  time; the final always() cleanup stays as the failure-path safety net.
- Make both servers hash passwords with the same scheme (SSHA-256), hashed
  server-side on write: MODIFY sends the password in cleartext and each server
  hashes it. OpenLDAP loads the pw-sha2 and ppolicy modules, sets
  olcPasswordHash {SSHA256} and enables the ppolicy hash-cleartext overlay on the
  mdb database; OpenDJ sets the Default Password Policy default storage scheme to
  Salted SHA-256. Each server hashes/verifies with its own implementation, so
  there is no cross-implementation hash-format dependency.
- Charts: render each operation as two adjacent, non-overlapping columns
  (OpenLDAP / OpenDJ) in different colors instead of two overlapping series, using
  an interleaved x-axis with zero-padded series and an explicit color palette
  (Mermaid xychart-beta has no grouped bars).
- Bump actions/cache@v4 -> @v5 and actions/upload-artifact@v4 -> @v7 (latest used
  elsewhere in this repo).

Validated locally with JMeter 5.6.3: the plan compiles, statistics.json carries
the READD label with ADMIN_CONNECT filtered out, summary.sh renders the two-column
charts, and actionlint passes.