docs: fix docs build and render the callback diagram as themed mermaid

[joshua-temple]

Problem

Two issues in the docs site, both from the recent refactor:

1. The docs build was broken on main. An earlier edit put a colon-space inside the unquoted YAML frontmatter description of internals/architecture.md, which the Astro build rejects. This check is not a required gate, so it slipped through.
2. reference/callbacks.md still carried an ASCII box-drawing diagram in a plain code fence, inconsistent with the mermaid diagrams the rest of the site uses. Its node boxes also rendered in mermaid's default lavender rather than the cascade palette.

Fix

• Quote the architecture frontmatter description so the site builds again.
• Convert the callback diagram to a mermaid flowchart.
• Brand the mermaid node fill, border, and text (mainBkg/nodeBorder/nodeTextColor) to cascade teal, matching the README badges, since the flowchart renderer keys node styling off those variables rather than the primary* ones.

Verification

npm run build succeeds (23 pages). Rendered the callback page in a browser and confirmed node fill #0E8B82, border #36D0C4, and teal edges, with no default-lavender remaining. No box-drawing characters left in any page.