test(upgrade): expand v4→v5 upgrade matrix with v4.3.x sub-cases

- Add HARPER_LEGACY_VERSION env var and isV43 flag so the CI matrix can
  activate version-specific test suites alongside the base upgrade suite
- Add suite 'v4.3.x: clustering→replication config rename' — verifies
  v5 starts cleanly when HARPER_SET_CONFIG carries the old clustering:
  key shape from v4.3.x and that all data survives the upgrade
- Add suite 'v4.3.x: NATS hub/leaf migration' — exercises the full
  hub/leaf clustering: block (routes, remotes, TLS, replyService) on
  a single-node v5 upgrade; multi-node NATS infra not required
- Add suite 'large audit log: v5 opens v4 system DB without OOM' —
  generates LARGE_AUDIT_RECORD_COUNT (500) upserts to grow the audit
  log, then verifies v5 opens the system DB and reads audit entries
  (CI-safe proxy for the 4 GB+ production case)
- Add suite 'v4 hdb_status GTM table after v4→v5 upgrade' — confirms
  the system.hdb_status table is queryable (or at least non-crashing)
  after upgrade, covering the GTM migration path

Closes #1187

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: github-actions[bot]