Firefox-based stealth scraper backend (interest check)

[feder-cr]

hey, before building it i wanted to check if there'd be interest in an optional Firefox-based stealth scraper backend under gpt_researcher/scraper/, parallel to firecrawl / browser / web_base_loader / tavily_extract.

motivation: a growing share of research-relevant pages sit behind Cloudflare, Akamai, Datadome, or hCaptcha. relevant open issues are #1685 (anybrowse MCP for Cloudflare), #1081 (Crunchbase Cloudflare block), #1602 (Firecrawl returns empty results), #1404 (self-hosted Firecrawl returns 0). a Firefox build with stealth patches at the C++ source level covers those cases without JS shims to detect.

the backend would wrap feder-cr/invisible_playwright, which drives a patched Firefox 150 (feder-cr/invisible_firefox, MPL-2, same license as Firefox upstream). selected when the user sets SCRAPER=invisible_firefox. optional dependency, only imported when selected. no change to defaults.

also opened a draft PR #1786 with an RFC stub in docs/docs/proposals/ so the proposal has somewhere concrete to land.