Mistake #19 (and BOOTSTRAP item #10) incorrectly stated the dev server is
unaffected. In practice, saving the file triggers Vite HMR which hits the
same SyntaxError and crashes the page with a 500. Confirmed in session where
sign_in_out.svelte used presenter_id_hint?: string | null.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>