feat(relayer): add k6 stress test harness

[ducnmm]

Summary

• add k6 relayer stress/load/spike harness with signed protected requests
• document k6 profiles and staging/local usage
• include k6 build in benchmark smoke workflow and support SUI_RPC_URL in sidecar

Verification

• npm test
• npm run k6:build
• npm audit --audit-level=moderate
• git diff --check

[ducnmm]

Closing as duplicate of #145 for MEM-30. #145 carries the same k6 relayer stress-test harness with less dependency/lockfile churn and a MEM-30-specific branch/title.