Mermaid rendering test (automated, deletable)

This is an automated rendering test (safe to delete). It embeds a Mermaid
diagram three ways, to see what Talkyard's CommonMark -> Google Caja sanitizer keeps,
and whether anything actually renders as a diagram.

1. Fenced code block tagged mermaid (what an author would naturally type):

graph TD
  A[Browser] --> B[caddy]
  B --> C[web]

2. Raw <pre class="mermaid"> block (what a Mermaid integration usually needs):

graph TD
  A[Browser] --> B[caddy]

3. Inline <svg> (could a pre-rendered diagram be pasted as SVG?):

svg kept?

End of test.