feat(sitemap): add XML sitemap generation module

Implement src/sitemap.rs with generate_sitemap() for SEO-compliant
XML sitemap generation. Follows the feed.rs pattern:

- SitemapEntry struct for URL metadata
- build_sitemap_xml() for XML construction
- xml_escape() for special character handling
- 5 unit tests covering single/multiple entries, lastmod, escaping

Module declared in main.rs but not yet integrated into pipeline.

src/main.rs

@@ -11,6 +11,7 @@ mod highlight;
 mod math;
 mod mermaid;
 mod render;
+mod sitemap;
 mod template_engine;
 
 use crate::content::{discover_nav, discover_sections, Content, ContentKind, NavItem};

src/sitemap.rs

@@ -0,0 +1,185 @@
+//! XML sitemap generation for SEO.
+
+use crate::config::SiteConfig;
+use crate::content::{Content, Section};
+use std::path::Path;
+
+/// A URL entry for the sitemap.
+pub struct SitemapEntry {
+    /// Absolute URL (e.g., "https://example.com/blog/post.html")
+    pub loc: String,
+    /// Optional last modification date in W3C format (YYYY-MM-DD)
+    pub lastmod: Option<String>,
+}
+
+/// Generate an XML sitemap from discovered content.
+///
+/// Includes:
+/// - Homepage
+/// - Section indices
+/// - Section items (posts, projects, etc.)
+/// - Standalone pages
+pub fn generate_sitemap(
+    sections: &[Section],
+    pages: &[Content],
+    config: &SiteConfig,
+    content_root: &Path,
+) -> String {
+    let base_url = config.base_url.trim_end_matches('/');
+    let mut entries = Vec::new();
+
+    // Homepage
+    entries.push(SitemapEntry {
+        loc: format!("{}/index.html", base_url),
+        lastmod: None,
+    });
+
+    // Sections and their items
+    for section in sections {
+        // Section index
+        entries.push(SitemapEntry {
+            loc: format!("{}/{}/index.html", base_url, section.name),
+            lastmod: section.index.frontmatter.date.clone(),
+        });
+
+        // Section items
+        if let Ok(items) = section.collect_items() {
+            for item in items {
+                let relative_path = item.output_path(content_root);
+                entries.push(SitemapEntry {
+                    loc: format!("{}/{}", base_url, relative_path.display()),
+                    lastmod: item.frontmatter.date.clone(),
+                });
+            }
+        }
+    }
+
+    // Standalone pages
+    for page in pages {
+        let relative_path = page.output_path(content_root);
+        entries.push(SitemapEntry {
+            loc: format!("{}/{}", base_url, relative_path.display()),
+            lastmod: page.frontmatter.date.clone(),
+        });
+    }
+
+    build_sitemap_xml(&entries)
+}
+
+/// Build the XML sitemap string from entries.
+fn build_sitemap_xml(entries: &[SitemapEntry]) -> String {
+    let mut urls = String::new();
+
+    for entry in entries {
+        urls.push_str("  <url>\n");
+        urls.push_str(&format!("    <loc>{}</loc>\n", xml_escape(&entry.loc)));
+        if let Some(ref date) = entry.lastmod {
+            urls.push_str(&format!("    <lastmod>{}</lastmod>\n", xml_escape(date)));
+        }
+        urls.push_str("  </url>\n");
+    }
+
+    format!(
+        r#"<?xml version="1.0" encoding="utf-8"?>
+<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
+{}
+</urlset>
+"#,
+        urls.trim_end()
+    )
+}
+
+/// Escape XML special characters.
+fn xml_escape(s: &str) -> String {
+    s.replace('&', "&amp;")
+        .replace('<', "&lt;")
+        .replace('>', "&gt;")
+        .replace('"', "&quot;")
+        .replace('\'', "&apos;")
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_xml_escape() {
+        assert_eq!(xml_escape("Hello & World"), "Hello &amp; World");
+        assert_eq!(xml_escape("<tag>"), "&lt;tag&gt;");
+        assert_eq!(xml_escape("\"quoted\""), "&quot;quoted&quot;");
+        assert_eq!(xml_escape("it's"), "it&apos;s");
+    }
+
+    #[test]
+    fn test_build_sitemap_xml_single_entry() {
+        let entries = vec![SitemapEntry {
+            loc: "https://example.com/index.html".to_string(),
+            lastmod: None,
+        }];
+
+        let xml = build_sitemap_xml(&entries);
+
+        assert!(xml.starts_with(r#"<?xml version="1.0" encoding="utf-8"?>"#));
+        assert!(xml.contains("<urlset xmlns="));
+        assert!(xml.contains("<url>"));
+        assert!(xml.contains("<loc>https://example.com/index.html</loc>"));
+        assert!(xml.contains("</urlset>"));
+        assert!(!xml.contains("<lastmod>")); // No lastmod when None
+    }
+
+    #[test]
+    fn test_build_sitemap_xml_with_lastmod() {
+        let entries = vec![SitemapEntry {
+            loc: "https://example.com/blog/post.html".to_string(),
+            lastmod: Some("2026-01-31".to_string()),
+        }];
+
+        let xml = build_sitemap_xml(&entries);
+
+        assert!(xml.contains("<loc>https://example.com/blog/post.html</loc>"));
+        assert!(xml.contains("<lastmod>2026-01-31</lastmod>"));
+    }
+
+    #[test]
+    fn test_build_sitemap_xml_multiple_entries() {
+        let entries = vec![
+            SitemapEntry {
+                loc: "https://example.com/index.html".to_string(),
+                lastmod: None,
+            },
+            SitemapEntry {
+                loc: "https://example.com/about.html".to_string(),
+                lastmod: Some("2026-01-15".to_string()),
+            },
+            SitemapEntry {
+                loc: "https://example.com/blog/index.html".to_string(),
+                lastmod: None,
+            },
+        ];
+
+        let xml = build_sitemap_xml(&entries);
+
+        // Count url elements
+        let url_count = xml.matches("<url>").count();
+        assert_eq!(url_count, 3);
+
+        // Verify all URLs present
+        assert!(xml.contains("https://example.com/index.html"));
+        assert!(xml.contains("https://example.com/about.html"));
+        assert!(xml.contains("https://example.com/blog/index.html"));
+    }
+
+    #[test]
+    fn test_build_sitemap_xml_escapes_special_chars() {
+        let entries = vec![SitemapEntry {
+            loc: "https://example.com/search?q=foo&bar=baz".to_string(),
+            lastmod: None,
+        }];
+
+        let xml = build_sitemap_xml(&entries);
+
+        // & should be escaped
+        assert!(xml.contains("&amp;"));
+        assert!(!xml.contains("?q=foo&bar")); // Raw & should not appear
+    }
+}