A cold code page renders off the main thread, so a cold forge serves more than ~220 a second #9630

From #9566. Measured on the in-process copy: a cold code page costs ~14 ms end to end (server-timing: highlight 1-10 ms, render 5-17 ms), the one main thread serves ~220 a second, and a stranger crawl at 500/s is refused ~56% for ~20 s. Idea: render the cold page (highlight + layout + encode) in a worker so the loop stays on time and the stranger budgets stop dividing. Done: bun bench/code-pages-at-rate.ts --rate 500 --seconds 20 --warm 0 --crawl 3000 refuses fewer strangers than the 5,574 baseline in bench/results/9566-cold-forge.md, with p95 stated.