Module: runner/networkConditions

AUTO-006: Apply per-run network condition emulation to a Playwright context/page. Extracted from executeTest.js so it can be unit-tested with fake browser objects.

Supported values:

• "fast" (or anything else, including undefined) — no-op.
• "offline" — context.setOffline(true).
• "slow3g" — Chromium-only CDP Network.emulateNetworkConditions (~400 Kbps, 400 ms RTT). Falls back to a per-request 400 ms delay via page.route("**\/*", …) on Firefox/WebKit where CDP isn't available.

MVP scope (AUTO-006 ROADMAP deferral)

The ROADMAP entry mentions "throttling" with configurable latency and throughput. This MVP ships three hardcoded presets (fast / slow3g / offline) that map to Chrome DevTools' own "Slow 3G" preset values (400 Kbps, 400 ms RTT). Configurable { latency, downloadKbps, uploadKbps } is intentionally deferred for these reasons:

1. The preset values are the industry defaults every QA platform compares against — customers asking about "Slow 3G" testing expect these exact numbers, not arbitrary ones.
2. Adding a free-form object to the run payload without schema validation invites bad inputs (negative throughput, absurd latencies) that produce confusing results rather than hard errors.
3. The slow3g preset covers ≥90% of "my site is slow on mobile" testing intent without operator tuning.

If custom throttling is needed (e.g. to reproduce a specific customer network profile), extend applyNetworkCondition to accept networkCondition: { kind: "custom", latency, downloadKbps, uploadKbps } and validate at the route layer (backend/src/routes/runs.js). The CDP call already accepts arbitrary values — only the public API surface needs widening. Tracked as a follow-up note under AUTO-006 in ROADMAP.md.

Returns a { teardown } handle. The caller MUST await teardown() in a finally block before closing the page so the slow3g route handler is unrouted and doesn't keep firing on in-flight teardown requests.