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

Cargo.toml

@@ -35,3 +35,6 @@ lightningcss = "1.0.0-alpha.70"
 katex-rs = "0.2.3"
 serde    = { version = "1", features = ["derive"] }
 toml     = "0.8"
+
+# Diagram rendering
+mermaid-rs-renderer = { git = "https://github.com/1jehuang/mermaid-rs-renderer", default-features = false }