Fix mermaid SPA: don't mark elements before mermaid.js loads
renderAll() was marking pre.mermaid elements with data-mermaid-init
even when the mermaid CDN script hadn't loaded yet. When mermaid
finally loaded and the MutationObserver fired, the elements were
already marked and skipped. Now renderAll() returns early if
mermaid is not yet defined.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>