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/main.rs

@@ -9,6 +9,7 @@ mod error;
 mod feed;
 mod highlight;
 mod math;
+mod mermaid;
 mod render;
 mod templates;
 

src/mermaid.rs

@@ -0,0 +1,43 @@
+//! Mermaid diagram rendering via mermaid-rs-renderer.
+//!
+//! Converts Mermaid diagram definitions to SVG at build-time.
+
+use mermaid_rs_renderer::RenderOptions;
+
+/// Render a Mermaid diagram to SVG.
+///
+/// # Arguments
+/// * `code` - The Mermaid diagram definition
+///
+/// # Returns
+/// The rendered SVG string, or an error message on failure.
+pub fn render_diagram(code: &str) -> Result<String, String> {
+    let opts = RenderOptions::modern();
+    mermaid_rs_renderer::render_with_options(code, opts).map_err(|e| e.to_string())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_simple_flowchart() {
+        let result = render_diagram("flowchart LR; A-->B-->C").unwrap();
+        assert!(result.contains("<svg"));
+        assert!(result.contains("</svg>"));
+    }
+
+    #[test]
+    fn test_sequence_diagram() {
+        let result = render_diagram("sequenceDiagram\n    Alice->>Bob: Hello").unwrap();
+        assert!(result.contains("<svg"));
+    }
+
+    #[test]
+    fn test_invalid_syntax_no_panic() {
+        // Should not panic, returns error
+        let result = render_diagram("invalid diagram syntax ???");
+        // May succeed with error node or fail gracefully
+        assert!(result.is_ok() || result.is_err());
+    }
+}

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.