0.25.1 — Drop unused Mermaid dependency

Removed

• mermaid devDependency and MermaidDiagram.svelte. The docs site stopped
  rendering Mermaid when the developer-reference chapters moved to
  docs/archive/ (off the non-recursive docs/*.md glob in chapters.ts). The
  only remaining ```mermaid fences live in those archived chapters, which no
  longer load, so the lazy-loaded mermaid package (~700kB) was unreachable.
  Removed the dependency, the MermaidDiagram component, and the mermaid
  handling in Docs.svelte. No consumer impact: mermaid was a
  devDependency and never shipped in the tarball.