feat(api/scrapeURL): engpicker integ

[mogery]

under an experimental flag for now

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Summary by cubic

Integrates Engpicker to adjust engine selection per domain behind an experimental flag, preferring tlsclient when safe. No default behavior change. Supports Linear ENG-4152.

• New Features
  • Added __experimental_engpicker option (default false).
  • Introduced queryEngpickerVerdict(hostname) using index RPC with a 250ms timeout; returns TlsClientOk/ChromeCdpRequired/Uncertain/Unknown (Unknown on errors or write-only).
  • When verdict is TlsClientOk and no forceEngine is set, boosts tlsclient engines in the fallback sort to rank above CDP; otherwise keeps current ordering.
  • buildFallbackList is now async; scrape loop updated to await it.

^{Written for commit 5195a5d. Summary will update automatically on new commits.}

[cubic-dev-ai]

No issues found across 4 files