Phase 1 of #883. Browser-mode path in AvitoScraper: __aenter__/__aexit__
manage a BrowserFetcher when scraper_fetch_mode=="browser"; _fetch_serp_html
fetches via browser with soft firewall-detect + IP-rotate. curl_cffi remains
the default — no prod behavior change. Pagination terminates via empty-parse.
Refs #883, #884