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.lock

@@ -41,6 +41,12 @@ version = "0.2.21"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "683d7910e743518b0e34f1186f92494becacb047c7b6bf616c96772180fef923"
 
+[[package]]
+name = "anyhow"
+version = "1.0.100"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a23eb6b1614318a8071c9b2521f36b424b2c83db5eb3a0fead4a6c0809af6e61"
+
 [[package]]
 name = "arraydeque"
 version = "0.5.1"
@@ -74,6 +80,15 @@ dependencies = [
  "wyz",
 ]
 
+[[package]]
+name = "block-buffer"
+version = "0.10.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3078c7629b62d3f0439517fa394996acacc5cbc91c5a20d8c658e77abd503a71"
+dependencies = [
+ "generic-array",
+]
+
 [[package]]
 name = "bon"
 version = "3.8.2"
@@ -178,6 +193,15 @@ dependencies = [
  "unicode-segmentation",
 ]
 
+[[package]]
+name = "cpufeatures"
+version = "0.2.17"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "59ed5838eebb26a2bb2e58f6d5b5316989ae9d08bab10e0e6d103e656d1b0280"
+dependencies = [
+ "libc",
+]
+
 [[package]]
 name = "crossbeam-deque"
 version = "0.8.6"
@@ -203,6 +227,16 @@ version = "0.8.21"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d0a5c400df2834b80a4c3327b3aad3a4c4cd4de0629063962b03235697506a28"
 
+[[package]]
+name = "crypto-common"
+version = "0.1.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "78c8292055d1c1df0cce5d180393dc8cce0abec0a7102adb6c7b1eef6016d60a"
+dependencies = [
+ "generic-array",
+ "typenum",
+]
+
 [[package]]
 name = "cssparser"
 version = "0.33.0"
@@ -235,6 +269,16 @@ dependencies = [
  "syn 2.0.114",
 ]
 
+[[package]]
+name = "dagre_rust"
+version = "0.0.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "be0e44f22d2a32979a1265aa1a70f6e2f4f43ec0d562d483a1c818f9a4121926"
+dependencies = [
+ "graphlib_rust",
+ "ordered_hashmap",
+]
+
 [[package]]
 name = "darling"
 version = "0.23.0"
@@ -297,6 +341,16 @@ dependencies = [
  "matches",
 ]
 
+[[package]]
+name = "digest"
+version = "0.10.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9ed9a281f7bc9b7576e61468ba615a66a5c8cfdff42420a70aa82701a3b1e292"
+dependencies = [
+ "block-buffer",
+ "crypto-common",
+]
+
 [[package]]
 name = "dtoa"
 version = "1.0.11"
@@ -351,6 +405,16 @@ version = "2.0.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e6d5a32815ae3f33302d95fdcb2ce17862f8c65363dcfd29360480ba1001fc9c"
 
+[[package]]
+name = "generic-array"
+version = "0.14.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "85649ca51fd72272d7821adaf274ad91c288277713d9c18820d8499a7ff69e9a"
+dependencies = [
+ "typenum",
+ "version_check",
+]
+
 [[package]]
 name = "getopts"
 version = "0.2.24"
@@ -383,6 +447,15 @@ dependencies = [
  "wasip2",
 ]
 
+[[package]]
+name = "graphlib_rust"
+version = "0.0.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b47de2c5685b5bb3689d6b0977f38959f3d7d82d3556627cfebbfaa42eda05ea"
+dependencies = [
+ "ordered_hashmap",
+]
+
 [[package]]
 name = "gray_matter"
 version = "0.2.9"
@@ -478,6 +551,17 @@ dependencies = [
  "wasm-bindgen",
 ]
 
+[[package]]
+name = "json5"
+version = "0.4.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "96b0db21af676c1ce64250b5f40f3ce2cf27e4e47cb91ed91eb6fe9350b430c1"
+dependencies = [
+ "pest",
+ "pest_derive",
+ "serde",
+]
+
 [[package]]
 name = "katex-rs"
 version = "0.2.3"
@@ -597,6 +681,22 @@ version = "2.7.6"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f52b00d39961fc5b2736ea853c9cc86238e165017a493d1d5c8eac6bdc4cc273"
 
+[[package]]
+name = "mermaid-rs-renderer"
+version = "0.1.2"
+source = "git+https://github.com/1jehuang/mermaid-rs-renderer#66b7d8c3f3152153666d589e789a143e87afcec5"
+dependencies = [
+ "anyhow",
+ "dagre_rust",
+ "graphlib_rust",
+ "json5",
+ "once_cell",
+ "regex",
+ "serde",
+ "serde_json",
+ "thiserror",
+]
+
 [[package]]
 name = "nrd-sh"
 version = "0.1.0"
@@ -605,6 +705,7 @@ dependencies = [
  "katex-rs",
  "lightningcss",
  "maud",
+ "mermaid-rs-renderer",
  "pulldown-cmark",
  "serde",
  "thiserror",
@@ -632,6 +733,12 @@ version = "1.21.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "42f5e15c9953c5e4ccceeb2e7382a716482c34515315f7b03532b8b4e8393d2d"
 
+[[package]]
+name = "ordered_hashmap"
+version = "0.0.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cb1ea499d242299d564879c8b375c11285f5a26d62d8010768909d6d099c4c5a"
+
 [[package]]
 name = "outref"
 version = "0.1.0"
@@ -693,6 +800,49 @@ version = "0.2.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "df94ce210e5bc13cb6651479fa48d14f601d9858cfe0467f43ae157023b938d3"
 
+[[package]]
+name = "pest"
+version = "2.8.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2c9eb05c21a464ea704b53158d358a31e6425db2f63a1a7312268b05fe2b75f7"
+dependencies = [
+ "memchr",
+ "ucd-trie",
+]
+
+[[package]]
+name = "pest_derive"
+version = "2.8.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "68f9dbced329c441fa79d80472764b1a2c7e57123553b8519b36663a2fb234ed"
+dependencies = [
+ "pest",
+ "pest_generator",
+]
+
+[[package]]
+name = "pest_generator"
+version = "2.8.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3bb96d5051a78f44f43c8f712d8e810adb0ebf923fc9ed2655a7f66f63ba8ee5"
+dependencies = [
+ "pest",
+ "pest_meta",
+ "proc-macro2",
+ "quote",
+ "syn 2.0.114",
+]
+
+[[package]]
+name = "pest_meta"
+version = "2.8.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "602113b5b5e8621770cfd490cfd90b9f84ab29bd2b0e49ad83eb6d186cef2365"
+dependencies = [
+ "pest",
+ "sha2",
+]
+
 [[package]]
 name = "phf"
 version = "0.11.3"
@@ -1121,6 +1271,17 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "sha2"
+version = "0.10.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a7507d819769d01a365ab707794a4084392c824f54a7a6a7862f8c3d0892b283"
+dependencies = [
+ "cfg-if",
+ "cpufeatures",
+ "digest",
+]
+
 [[package]]
 name = "shlex"
 version = "1.3.0"
@@ -1452,6 +1613,18 @@ dependencies = [
  "tree-sitter-language",
 ]
 
+[[package]]
+name = "typenum"
+version = "1.19.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "562d481066bde0658276a35467c4af00bdc6ee726305698a55b86e61d7ad82bb"
+
+[[package]]
+name = "ucd-trie"
+version = "0.1.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2896d95c02a80c6d6a5d6e953d479f5ddf2dfdb6a244441010e373ac0fb88971"
+
 [[package]]
 name = "unicase"
 version = "2.9.0"

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 }

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.