Add k6 stress tests for MemWal relayer

[ducnmm]

Summary

• add a bundled k6 harness for relayer health, remember, recall, ask, and restore flows
• cover smoke, load, stress, spike, and health-only profiles with custom latency/error/timeout metrics
• document local/staging usage and note that live stress execution should wait for the Mysten RPC endpoint
• make the TS sidecar honor SUI_RPC_URL so custom/internal RPC config applies consistently

Verification

• npm run k6:build
• ./node_modules/.bin/tsc --noEmit --skipLibCheck --target ES2022 --module NodeNext --moduleResolution NodeNext bench-recall-latency.ts k6/relayer.ts
• npm test
• cargo check

Live k6 execution was not run because we do not yet have the mysten-rpc endpoint for the relayer environment.

Linear: MEM-30