Firefox-based stealth option (interest check)

[feder-cr]

hey, before building it i wanted to check if there'd be interest in an optional Firefox-based stealth backend for the Playwright launch path, parallel to the existing Chromium setup. selected via config flag, 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 their 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. issues against the backend route to feder-cr/invisible_playwright.

also opened a draft PR #905 with an RFC stub in docs/ so the proposal has somewhere concrete to land.

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可选的 Firefox 后端，作为 Chromium 路径的并行选项。详细方案见 PR 中的英文说明。