feat(mermaid): add mermaid-rs-renderer diagram module

Add src/mermaid.rs with render_diagram() wrapper around
mermaid-rs-renderer for build-time Mermaid-to-SVG conversion.

- Use mermaid-rs-renderer git dependency (SVG-only, no PNG)
- Configure with RenderOptions::modern() theme
- Include unit tests for flowchart and sequence diagrams

src/render.rs

@@ -1,6 +1,6 @@
 //! Markdown to HTML rendering via pulldown-cmark with syntax highlighting.
 
-use crate::highlight::{Language, highlight_code};
+use crate::highlight::{highlight_code, Language};
 use pulldown_cmark::{CodeBlockKind, Event, Options, Parser, Tag, TagEnd};
 
 /// Render markdown content to HTML with syntax highlighting.