feat(content): add filesystem-driven nav discovery

Add NavItem struct and discover_nav() function to scan content
directory and automatically build navigation from:
- Top-level .md files (pages)
- Directories with _index.md (sections)

Navigation is sorted by frontmatter weight, then alphabetically.
Custom nav_label field allows overriding title in nav menu.

Includes 5 unit tests covering page/section discovery, weight
ordering, and nav_label support.

src/content.rs

@@ -18,6 +18,17 @@ pub enum ContentKind {
     Project,
 }
 
+/// A navigation menu item discovered from the filesystem.
+#[derive(Debug, Clone)]
+pub struct NavItem {
+    /// Display label (from nav_label or title)
+    pub label: String,
+    /// URL path (e.g., "/blog/index.html" or "/about.html")
+    pub path: String,
+    /// Sort order (lower = first, default 50)
+    pub weight: i64,
+}
+
 /// Parsed frontmatter from a content file.
 #[derive(Debug)]
 pub struct Frontmatter {
@@ -25,10 +36,12 @@ pub struct Frontmatter {
     pub description: Option<String>,
     pub date: Option<String>,
     pub tags: Vec<String>,
-    /// For project cards: sort order
+    /// Sort order for nav and listings
     pub weight: Option<i64>,
     /// For project cards: external link
     pub link_to: Option<String>,
+    /// Custom navigation label (defaults to title)
+    pub nav_label: Option<String>,
 }
 
 /// A content item ready for rendering.
@@ -120,6 +133,7 @@ fn parse_frontmatter(path: &Path, parsed: &gray_matter::ParsedEntity) -> Result<
     let date = pod.get("date").and_then(|v| v.as_string().ok());
     let weight = pod.get("weight").and_then(|v| v.as_i64().ok());
     let link_to = pod.get("link_to").and_then(|v| v.as_string().ok());
+    let nav_label = pod.get("nav_label").and_then(|v| v.as_string().ok());
 
     // Handle nested taxonomies.tags structure
     let tags = if let Some(taxonomies) = pod.get("taxonomies") {
@@ -147,5 +161,167 @@ fn parse_frontmatter(path: &Path, parsed: &gray_matter::ParsedEntity) -> Result<
         tags,
         weight,
         link_to,
+        nav_label,
     })
 }
+
+/// Discover navigation items from the content directory structure.
+///
+/// Rules:
+/// - Top-level `.md` files (except `_index.md`) become nav items (pages)
+/// - Directories containing `_index.md` become nav items (sections)
+/// - Items are sorted by weight (lower first), then alphabetically by label
+pub fn discover_nav(content_dir: &Path) -> Result<Vec<NavItem>> {
+    let mut nav_items = Vec::new();
+
+    // Read top-level entries in content directory
+    let entries = fs::read_dir(content_dir).map_err(|e| Error::ReadFile {
+        path: content_dir.to_path_buf(),
+        source: e,
+    })?;
+
+    for entry in entries.filter_map(|e| e.ok()) {
+        let path = entry.path();
+
+        if path.is_file() {
+            // Top-level .md file (except _index.md) → page nav item
+            if path.extension().is_some_and(|ext| ext == "md") {
+                let file_name = path.file_name().and_then(|n| n.to_str()).unwrap_or("");
+                if file_name != "_index.md" {
+                    let content = Content::from_path(&path, ContentKind::Page)?;
+                    let slug = path.file_stem().and_then(|s| s.to_str()).unwrap_or("page");
+                    nav_items.push(NavItem {
+                        label: content
+                            .frontmatter
+                            .nav_label
+                            .unwrap_or(content.frontmatter.title),
+                        path: format!("/{}.html", slug),
+                        weight: content.frontmatter.weight.unwrap_or(50),
+                    });
+                }
+            }
+        } else if path.is_dir() {
+            // Directory with _index.md → section nav item
+            let index_path = path.join("_index.md");
+            if index_path.exists() {
+                let content = Content::from_path(&index_path, ContentKind::Section)?;
+                let dir_name = path
+                    .file_name()
+                    .and_then(|n| n.to_str())
+                    .unwrap_or("section");
+                nav_items.push(NavItem {
+                    label: content
+                        .frontmatter
+                        .nav_label
+                        .unwrap_or(content.frontmatter.title),
+                    path: format!("/{}/index.html", dir_name),
+                    weight: content.frontmatter.weight.unwrap_or(50),
+                });
+            }
+        }
+    }
+
+    // Sort by weight, then alphabetically by label
+    nav_items.sort_by(|a, b| a.weight.cmp(&b.weight).then_with(|| a.label.cmp(&b.label)));
+
+    Ok(nav_items)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::fs;
+
+    fn create_test_dir() -> tempfile::TempDir {
+        tempfile::tempdir().expect("failed to create temp dir")
+    }
+
+    fn write_frontmatter(path: &Path, title: &str, weight: Option<i64>, nav_label: Option<&str>) {
+        let mut content = format!("---\ntitle: \"{}\"\n", title);
+        if let Some(w) = weight {
+            content.push_str(&format!("weight: {}\n", w));
+        }
+        if let Some(label) = nav_label {
+            content.push_str(&format!("nav_label: \"{}\"\n", label));
+        }
+        content.push_str("---\n\nBody content.");
+        fs::write(path, content).expect("failed to write test file");
+    }
+
+    #[test]
+    fn test_discover_nav_finds_pages() {
+        let dir = create_test_dir();
+        let content_dir = dir.path();
+
+        // Create top-level page
+        write_frontmatter(&content_dir.join("about.md"), "About Me", None, None);
+
+        let nav = discover_nav(content_dir).expect("discover_nav failed");
+        assert_eq!(nav.len(), 1);
+        assert_eq!(nav[0].label, "About Me");
+        assert_eq!(nav[0].path, "/about.html");
+    }
+
+    #[test]
+    fn test_discover_nav_finds_sections() {
+        let dir = create_test_dir();
+        let content_dir = dir.path();
+
+        // Create section directory with _index.md
+        let blog_dir = content_dir.join("blog");
+        fs::create_dir(&blog_dir).expect("failed to create blog dir");
+        write_frontmatter(&blog_dir.join("_index.md"), "Blog", None, None);
+
+        let nav = discover_nav(content_dir).expect("discover_nav failed");
+        assert_eq!(nav.len(), 1);
+        assert_eq!(nav[0].label, "Blog");
+        assert_eq!(nav[0].path, "/blog/index.html");
+    }
+
+    #[test]
+    fn test_discover_nav_excludes_root_index() {
+        let dir = create_test_dir();
+        let content_dir = dir.path();
+
+        // Create _index.md at root (should be excluded from nav)
+        write_frontmatter(&content_dir.join("_index.md"), "Home", None, None);
+        write_frontmatter(&content_dir.join("about.md"), "About", None, None);
+
+        let nav = discover_nav(content_dir).expect("discover_nav failed");
+        assert_eq!(nav.len(), 1);
+        assert_eq!(nav[0].label, "About");
+    }
+
+    #[test]
+    fn test_discover_nav_sorts_by_weight() {
+        let dir = create_test_dir();
+        let content_dir = dir.path();
+
+        write_frontmatter(&content_dir.join("about.md"), "About", Some(30), None);
+        write_frontmatter(&content_dir.join("contact.md"), "Contact", Some(10), None);
+        write_frontmatter(&content_dir.join("blog.md"), "Blog", Some(20), None);
+
+        let nav = discover_nav(content_dir).expect("discover_nav failed");
+        assert_eq!(nav.len(), 3);
+        assert_eq!(nav[0].label, "Contact"); // weight 10
+        assert_eq!(nav[1].label, "Blog"); // weight 20
+        assert_eq!(nav[2].label, "About"); // weight 30
+    }
+
+    #[test]
+    fn test_discover_nav_uses_nav_label() {
+        let dir = create_test_dir();
+        let content_dir = dir.path();
+
+        write_frontmatter(
+            &content_dir.join("about.md"),
+            "About The Author",
+            None,
+            Some("About"),
+        );
+
+        let nav = discover_nav(content_dir).expect("discover_nav failed");
+        assert_eq!(nav.len(), 1);
+        assert_eq!(nav[0].label, "About"); // Uses nav_label, not title
+    }
+}