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 (invisible-playwright on PyPI), which drives a patched Firefox built from feder-cr/firefox_antidetect_patch (MPL-2, same license as Firefox upstream) and pulled in through feder-cr/invisible_core. 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.

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.

[feder-cr]

@NanmiCoder 友情提醒一下这个提案,依然是可选项不影响默认流程,如果不需要的话可以直接关闭。

[feder-cr]

Fixed a stale reference in here - invisible_firefox was a separate GUI profile manager and it got retired, repo and PyPI project both gone as of a few days ago. The patched Firefox itself is built in firefox_antidetect_patch and invisible_playwright pulls the binary through invisible_core, so the actual proposal - opt-in Playwright backend, same BrowserContext, same login state - doesn't change. Also dropped the tracking-discussion link since that issue number doesn't resolve on this repo anymore. Updated the doc and description to only point at what's actually there today.