sampling_parallel(n_chains=...) spawns one plain-JS Web Worker per chain -- no Pyodide in
them, just stanwasm loaded fresh in each -- so multiple
chains run concurrently. This page runs PyScript on the main thread (not a Worker, since PyScript's Worker
mode needs cross-origin-isolation headers this static site doesn't set), so the Workers
sampling_parallel spawns aren't nested inside another Worker here, unlike the
JupyterLite/marimo demos -- just ordinary Workers off the main
thread, an easier case.