improvement(desktop-browser): let a page finish rendering before observing an action - #9985

Summary

• Before an action's observation, wait until the page has gone 150ms without a structural DOM change (nodes or text; attribute churn such as animations is ignored) and finished loading, capped at 2s. A page already quiet costs one probe
• Why: in production-loop traces, when the model re-snapshotted right after an action that already returned a snapshot, the page had changed in 72 of 73 cases — the observation averaged 14 outline lines (often only the logo mid route change) against 73 a round later — so the agent spent a round re-observing or acted on a half-rendered page
• The watcher is a single document-wide MutationObserver installed by the shared page helpers

Type of Change

• Improvement

Testing

• Red/green driver tests: a page still rendering is observed only once it goes quiet, an already quiet page costs one probe, a page that never settles is still observed within the cap; 551 browser-agent tests pass, desktop type-check clean
• Production-loop A/B (Opus 5.5, Sim's Electron driver; REAL ×2 seeds, WebGames, capbench; 302 paired episodes, arms concurrent): success 220 → 221, rounds/episode −6%, wall −10%, cost −10%, failed tool calls −21% on episodes both arms passed. The REAL losses are model choices or renderer crashes and agent-process exits that hit both arms equally during that run

Checklist

• Code follows project style guidelines
• Self-reviewed my changes
• Tests added/updated and passing (new tests pass the test-audit authoring gate)
• No new warnings introduced
• I confirm that I have read and agree to the terms outlined in the Contributor License Agreement (CLA)