feat(qss-stress): toxiproxy harness, precise wire-fault hooks, jest config

[holmesworcester]

Summary

Lands the QSS stress tooling on 7.1.0 — the toxiproxy harness, wire-fault hook, jest config, and helper scripts. Scenario specs (the proofs-of-concept used during the QSS investigation) are intentionally left out of this PR and will land separately on top.

What's in

Harness:

• harness.ts — bootQssHarness() / bootMemberHarness() boot the QSS path of the Nest app behind toxiproxy. Per-test proxy isolation: every call allocates a unique proxy name and free local port and tears it down on shutdown, so multiple stress runs can execute in parallel under jest's --maxWorkers without colliding.
• precise-faults.ts — installWireFaultHooks() monkey-patches qssClient.sendMessage to inject precise per-message faults (drop / delay / error / busy-wait pause) keyed by WebsocketEvents type. Helpers: scheduleOutageAfter, oneShot, busyWaitPause. Use it when network-level toxiproxy chaos is too coarse and a bug needs message-type granularity.
• toxiproxy.ts — tiny zero-dep client for the toxiproxy admin API with retry-with-backoff for transient 503/timeout/ECONNRESET. The admin API is single-threaded; under concurrent load it returns transient errors that are safe to retry. Also splits rawRequest from request and surfaces HTTP status on errors.
• docker-compose.yml — toxiproxy sidecar (host networking on Linux).
• README.md — usage and configuration.

Test plumbing:

• packages/backend/jest.stress.config.js — dedicated jest config for the stress harness (ts-jest ESM, type-diagnostics off, longer timeout, scoped testRegex).
• packages/backend/package.json — test-stress-qss script + testPathIgnorePatterns so *.stress.spec.ts is excluded from default test/test-ci.

Scripts:

• Root package.json — start:qss:toxi / stop:qss:toxi convenience wrappers around docker compose for the toxiproxy sidecar.

Not in this PR

Scenario specs (scenarios/*.stress.spec.ts), the chaos profile catalog (fuzz.ts), invariant probes (invariants.ts), and FINDINGS.md are part of the investigation work and will land in follow-ups.

Test plan

• npm run start:qss (existing)
• npm run start:qss:toxi brings up the toxiproxy sidecar
• cd packages/backend && npm run test-stress-qss — currently a no-op since no *.stress.spec.ts exist on this branch yet; should run jest cleanly with no test files matched
• npm run test / npm run test-ci does not pick up anything under src/nest/qss/stress/ (excluded via testPathIgnorePatterns)
• npm run stop:qss:toxi