Add functional health probes and safe lifecycle

[Osvaldo Andrade (osvaldoandrade)]

Summary

• Split process health into /livez, /readyz, /startupz, and /raftz.
• Add lifecycle drain state, identity-lease readiness checks, and shutdown leadership handoff.
• Add a storage write gate so new writes stop during drain while committed Raft entries can still apply.
• Update Helm probes, lifecycle config, and headless service readiness behavior.

Validation

• go test ./...
• helm template test dist/helm/cefas
• Render check for /livez, /readyz, /startupz, lifecycle config, and headless publishNotReadyAddresses.
• RF=3 docker-compose benchmark: /tmp/cefas-bench/functional-health-lifecycle-rf3-standard/summary.md
  • write_only: 56,163,500 writes, 186,339/s, 0 errors
  • read_only: 32,777,269 reads, 109,251/s, 0 errors
  • mixed write: 29,371,000 writes, 97,761/s, 0 errors
  • mixed read: 15,909,289 reads, 52,954/s, 0 errors

Closes #571
Closes #579
Closes #580
Closes #581