feat(qss-stress): consolidate harness — precise faults, parallel proxies, retry

[holmesworcester]

Summary

Wraps up the QSS stress harness with the last pieces from the investigation work plus the parallelism / robustness improvements that weren't part of the earlier per-peer-chaos / leave-switch / pause-primitive merges.

What's in

Infra:

• 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 the bug needs message-type granularity.
• harness.ts — per-test proxy isolation. Every bootQssHarness() / bootMemberHarness() call allocates a unique proxy name and a free local port, and deletes the proxy on shutdown. Lets multiple stress runs execute in parallel under jest's --maxWorkers without colliding on a shared qss proxy.
• toxiproxy.ts — retry-with-backoff for transient 503/timeout/ECONNRESET on the toxiproxy admin API. 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.

Scenarios:

• scenarios/flap-stalls-create.stress.spec.ts — rapid proxy enable/disable flaps during preCreate and duringAuthSync. Surfaces stalls in the sign-in / auth-conn state machines under churn.
• scenarios/wire-fault-acks.stress.spec.ts — per-message ack-handling sweep using installWireFaultHooks. Demonstrates wire-fault hook usage.

Scripts:

• Root: start:qss:toxi / stop:qss:toxi convenience wrappers around docker compose for the toxiproxy sidecar.
• Backend: test-stress-qss script for running the dedicated jest config. testPathIgnorePatterns excludes stress/ from default test/test-ci so CI is unaffected.

Test plan

• docker compose up toxiproxy sidecar
• npm run start:qss (existing)
• cd packages/backend && npm run test-stress-qss — verify the suite runs end-to-end against a real QSS server
• Confirm npm run test/test-ci does not pick up *.stress.spec.ts (excluded via testPathIgnorePatterns)

[holmesworcester]

Superseded by TryQuiet/quiet#3219, which lands just the toxiproxy tooling (harness, precise-faults, toxiproxy client, jest config, scripts) on 7.1.0. The scenario specs / fuzz catalog / invariants / FINDINGS will land separately. Closing this.