feat: add Atom feed generation

- src/feed.rs: New module with generate_atom_feed() that produces
  Atom 1.0 XML from blog posts. Uses config.base_url for absolute
  entry URLs. Includes xml_escape() helper.

- src/main.rs: Wire mod feed and call generate_feed() after blog
  index generation. Outputs to public/feed.xml.

- src/templates.rs: Add <link rel="alternate" type="application/atom+xml">
  autodiscovery link to page head using config.base_url.

Feed includes title, author, updated timestamp, and entries with
title, link, id, updated, and summary for each blog post.

src/feed.rs

@@ -0,0 +1,93 @@
+//! Atom feed generation.
+
+use crate::config::SiteConfig;
+use crate::content::Content;
+
+/// Generate an Atom 1.0 feed from blog posts.
+pub fn generate_atom_feed(posts: &[Content], config: &SiteConfig) -> String {
+    let base_url = config.base_url.trim_end_matches('/');
+
+    // Use the most recent post date as feed updated time, or fallback
+    let updated = posts
+        .first()
+        .and_then(|p| p.frontmatter.date.as_ref())
+        .map(|d| format!("{}T00:00:00Z", d))
+        .unwrap_or_else(|| "1970-01-01T00:00:00Z".to_string());
+
+    let mut entries = String::new();
+    for post in posts {
+        let post_url = format!("{}/blog/{}.html", base_url, post.slug);
+        let post_date = post
+            .frontmatter
+            .date
+            .as_ref()
+            .map(|d| format!("{}T00:00:00Z", d))
+            .unwrap_or_else(|| "1970-01-01T00:00:00Z".to_string());
+
+        let summary = post
+            .frontmatter
+            .description
+            .as_ref()
+            .map(|s| xml_escape(s))
+            .unwrap_or_default();
+
+        entries.push_str(&format!(
+            r#"  <entry>
+    <title>{}</title>
+    <link href="{}" rel="alternate"/>
+    <id>{}</id>
+    <updated>{}</updated>
+    <summary>{}</summary>
+  </entry>
+"#,
+            xml_escape(&post.frontmatter.title),
+            post_url,
+            post_url,
+            post_date,
+            summary,
+        ));
+    }
+
+    format!(
+        r#"<?xml version="1.0" encoding="utf-8"?>
+<feed xmlns="http://www.w3.org/2005/Atom">
+  <title>{}</title>
+  <link href="{}" rel="alternate"/>
+  <link href="{}/feed.xml" rel="self"/>
+  <id>{}/</id>
+  <updated>{}</updated>
+  <author>
+    <name>{}</name>
+  </author>
+{}
+</feed>
+"#,
+        xml_escape(&config.title),
+        base_url,
+        base_url,
+        base_url,
+        updated,
+        xml_escape(&config.author),
+        entries,
+    )
+}
+
+/// 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;");
+    }
+}

src/main.rs

@@ -6,6 +6,7 @@ mod config;
 mod content;
 mod css;
 mod error;
+mod feed;
 mod highlight;
 mod render;
 mod templates;
@@ -45,6 +46,9 @@ fn run() -> Result<()> {
     posts.sort_by(|a, b| b.frontmatter.date.cmp(&a.frontmatter.date));
     generate_blog_index(output_dir, &posts, &config)?;
 
+    // 2b. Generate Atom feed
+    generate_feed(output_dir, &posts, &config)?;
+
     // 3. Process standalone pages (about, collab)
     process_pages(content_dir, output_dir, &config)?;
 
@@ -122,6 +126,22 @@ fn generate_blog_index(
     Ok(())
 }
 
+/// Generate the Atom feed
+fn generate_feed(output_dir: &Path, posts: &[Content], config: &config::SiteConfig) -> Result<()> {
+    let out_path = output_dir.join("feed.xml");
+    eprintln!("generating: {}", out_path.display());
+
+    let feed_xml = feed::generate_atom_feed(posts, config);
+
+    fs::write(&out_path, feed_xml).map_err(|e| Error::WriteFile {
+        path: out_path.clone(),
+        source: e,
+    })?;
+
+    eprintln!("  → {}", out_path.display());
+    Ok(())
+}
+
 /// Process standalone pages in content/ (about.md, collab.md)
 fn process_pages(content_dir: &Path, output_dir: &Path, config: &config::SiteConfig) -> Result<()> {
     for name in ["about.md", "collab.md"] {

src/templates.rs

@@ -196,6 +196,7 @@ fn base_layout(title: &str, content: Markup, page_path: &str, config: &SiteConfi
                 meta name="viewport" content="width=device-width, initial-scale=1";
                 title { (title) " | " (config.title) }
                 link rel="canonical" href=(canonical_url);
+                link rel="alternate" type="application/atom+xml" title="Atom Feed" href=(format!("{}/feed.xml", config.base_url.trim_end_matches('/')));
                 link rel="stylesheet" href=(format!("{}/style.css", prefix));
             }
             body {