Bakes the proven 2026-07-06 cold-vs-warm pause-hook flow into
scripts/device-testing/ as a first-class, reproducible `cold-fetch-ab`
scenario, upstreaming the scratchpad driver.
- fetchgate.py: FETCH_GATE pause/resume/query helpers through the guarded
adb wrapper, with ordered-broadcast read-back parsing (paused=[...]).
- scenarios.cold_fetch_ab: pre-arm halt -> detect sign-in (sync all
breadcrumb) -> confirm halt (prefetch skipped) -> wait for header sync ->
measure cold opens -> resume -> measure warm opens. ALWAYS resumes on exit
(finally), even on error -- never leaves fetch paused.
- report.render_cold_fetch_ab: gate summary, cold/warm tables, cold-vs-warm
delta, connect=0ms reuse proof, throttle signature.
- Portability (subsumes #392): file-based uiautomator dump (not /dev/tty),
UTF-8 adb decode + PYTHONUTF8/console I/O, openMessage-breadcrumb readiness,
row-selection hardening (skip non-message rows).
The pause hook is debug-build-only (#393/#395), so the scenario needs a debug
APK. Automated validation: mocked unittest coverage (adb/breadcrumbs/gate) for
the helpers and the A/B scenario incl. restore-on-error, plus a --dry-run path
exercised end-to-end through perf_harness.main. A full on-device run is a
follow-up. Dev-tooling only; no app/src changes.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
JMR-devandClaude Opus 4.8
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