Remove mermaid rendering from the design bundle

What does this MR do and why?

I want the design bundle to stop shipping mermaid because sites now draw diagrams as SVG with the diagram kit, and a search of the group's default branches finds no project outside the reference loading mermaid-init.js.

• Removed. js/mermaid-init.js, the vendored renderer under design/vendor/mermaid/, and the .mermaid rules in chrome/lab/mermaid.css.
• Kept. The .mermaid-card rules, byte for byte, still imported by the lab chrome, and the anchors.js handling of details.mermaid-card. Two live sites that load the chrome unpinned wrap SVG diagrams in it.
• Pages. The workspaces-airgap example carries the two SVG diagrams its source project draws, moved onto the theme tokens. The showcase's mermaid card becomes a Diagrams section drawn from the kit's process template.
• Docs. Component lists, chrome READMEs, the site's design and template pages, the Astro import template and the lab template stop offering mermaid. The bans and redraw method stay.

A page pinned to v2.5.0 still loads mermaid-init.js from that release's bundle.

Standards affected

design: a check.sh comment and the unasserted: note change. No assertion, tier or status changes.

Exemption implications

None.

Test plan

• just guard, just validate, just check, just build and just agent pass.
• The built chrome/lab/mermaid.css matches lines 37-86 on main; a lightningcss bundle of the lab chrome holds 7 .mermaid-card rules and no .mermaid rule.
• The docs template builds with its placeholders filled, and astro check reports 0 errors.
• retoken.py --check finds no unmapped color in either example diagram. Its layout findings (width, grid, type sizes) wait for a redraw to the kit.
• The showcase and the example render at 1440 in light and dark, every label and arrow readable.
• Not tested: the live sites after a deploy, other viewports, Safari, Firefox.

An AI tool wrote this change and ran the checks above.