fix: load balancer panics, stealth flag, shadow DOM polyfill

Fixes h4ckf0r0day#1: replace unwrap() panics in run_multi_worker_serve with match on the Result.
Fixes h4ckf0r0day#2: route /json round-robin via worker_port instead of hardcoded port+1.
Fixes h4ckf0r0day#3: return 502 Bad Gateway when a worker is unreachable instead of silently dropping the connection.
Fixes h4ckf0r0day#5: thread --stealth through start_with_options -> CdpContext -> BrowserContext::with_options so the flag actually has an effect in serve mode (was let _ before). Log message now reflects the actual capability based on the stealth cargo feature being compiled in.
Fixes h4ckf0r0day#15: try/catch the parentNode assignment in the attachShadow polyfill so it doesn't throw a TypeError against the read-only Node.parentNode getter (Cloudflare Turnstile triggers this path).

Update release.yml