The previous test only checked if childNodes.length hit zero. With
replaceChildren that never happens — but the flash is still visible
because the SSR DOM is replaced with different reactive DOM.
New test captures SSR textContent before JS boots, watches for any
change via MutationObserver. Now correctly fails:
"text changed — ssr:(div (~tw :tokens... → hydrated:..."
This proves the flash: island hydration replaces SSR DOM wholesale.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>