repo: dripnex/plugin-mermaid — extract mermaid diagrams

[tomymaritano]

Context

src/renderer/plugins/mermaid.tsx (175 LOC) becomes a satellite. Mermaid is a heavy dep — extracting it also removes it from the core bundle (startup win).

Acceptance criteria

• Repo from plugin-template; npm @dripnex/plugin-mermaid; runtime id dripnex-mermaid
• Block renderer for ```mermaid fences via plugin-api; lazy-loads the mermaid lib inside the plugin
• Hardcoded mermaid.tsx deleted; core renderer bundle size reduction measured and recorded in the PR
• Toggleable; identical rendering (e2e block assertion)

Depends on

Markdown extension points (Q1); plugin-math (pattern proof).