detectorPerf test pages

Manual test pages for the detectorPerf C-S-S feature, which measures on-device detector execution time and reports threshold-crossing counts through EventHub telemetry.

Seeing results. On debug builds, every page here renders a live overlay (bottom right) fed by the feature's debug-only detectorPerfDebugStats broadcast: exact per-detector runs / total / worst timings, severe crossings, and the last recorded run. Production builds broadcast nothing — there the results channels are pixel logs and breakage-report payloads.

Typical page — realistic article-style page (~1,700 DOM elements, around the median for real websites) with no adwall-adjacent content. Use it as the baseline: normal detector cost, no matches, no severe events expected.
Heavy page — DOM stress page with URL-drivable load (?paragraphs=N&iframes=N&framesrc=real&adwall=1&next=…). Grow the DOM until detector scans get slow and watch timings climb toward the configured threshold edges; a run crossing the highest edge fires the immediate severe event.

Per-frame measurement note: detectors are top-frame-only by default (runConditions default to context: { top: true }) and no current config opts any detector into subframes, so frame-level timings cannot be observed today. If a config ever enables frame detectors, a dedicated cross-origin frames page would be worth adding back (one existed briefly; removed as untestable).

Threshold edges come from remote config (detectorPerf feature settings). For manual threshold experiments, point the platform's config override at a test config with lowered edges — a near-zero severe edge makes every scan cross it deterministically.