docs(proposal): Firefox-based stealth option

[feder-cr]

opening as draft to check interest before building the launch helper switch.

would an optional Firefox-based stealth backend be in scope for the Playwright launch path, parallel to the existing Chromium setup? selected via config, no change to defaults.

motivation: saved-login plus Playwright is the project's core technique, but sessions still get flagged by platform fingerprint checks over time. a second engine option with stealth patches at the C++ source level gives operators a fallback when Chromium-based sessions start getting banned.

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). drop-in compatible with the existing playwright.async_api usage, same BrowserContext, same login state, same JS expression evaluation.

this PR only adds an RFC stub in docs/ so the proposal has somewhere concrete to land. tracking discussion: #904

if the answer is "not in scope" i'll close it without noise.

[feder-cr]

@NanmiCoder 这个隐身 Firefox 引擎选项有兴趣吗?opt-in, no change to the default flow. happy to close if not useful.