fix: TOC header class, numbering config, and Lua defaults

- Add class='toc' to TOC heading in HTML export by detecting the
  configured toc_header value in render_heading()
- Add toc_header field to HtmlRenderer and wire it through
  render_page_html()
- Add toc_header, toc_header_level, html_header_numbering, and
  html_header_numbering_sym to Lua config defaults so users can
  discover and set them

crates/nuwiki-core/src/render/html.rs

@@ -59,6 +59,10 @@ pub struct HtmlRenderer {
     /// vimwiki `html_header_numbering_sym`: a symbol appended after the
     /// section number (e.g. `.` → `1.2. Heading`). Empty by default.
     header_numbering_sym: String,
+    /// vimwiki `toc_header`: the heading text that identifies the TOC
+    /// section. When a heading matches this value (case-insensitive), it
+    /// gets `class="toc"` in HTML export.
+    toc_header: String,
     /// vimwiki `valid_html_tags`: inline HTML tag names passed through to
     /// export verbatim (instead of escaping `<`/`>`). Empty = escape all.
     /// `span` is always allowed so colour spans render.
@@ -90,6 +94,7 @@ impl HtmlRenderer {
             color_template: "<span style=\"__STYLE__\">__CONTENT__</span>".to_string(),
             header_numbering: 0,
             header_numbering_sym: String::new(),
+            toc_header: String::new(),
             valid_html_tags: Vec::new(),
             emoji_enable: false,
             text_ignore_newline: true,
@@ -183,6 +188,13 @@ impl HtmlRenderer {
         self.header_numbering_sym = sym.into();
         self
     }
+
+    /// Set the vimwiki TOC heading text. When a heading matches this value
+    /// (case-insensitive) it gets `class="toc"` in HTML export.
+    pub fn with_toc_header(mut self, header: impl Into<String>) -> Self {
+        self.toc_header = header.into();
+        self
+    }
 }
 
 /// Running state for vimwiki-style HTML section numbering
@@ -314,15 +326,27 @@ impl HtmlRenderer {
     /// pass `""` for no numbering.
     fn render_heading(&self, n: &HeadingNode, number: &str, w: &mut dyn Write) -> io::Result<()> {
         let level = n.level.clamp(1, 6);
-        let class = if n.centered {
-            " class=\"centered\""
-        } else {
-            ""
-        };
-        // `id` is the heading's plain text (vimwiki's anchor scheme), so
-        // `[[Page#Heading]]` and TOC `#Heading` links land here. Matches the
-        // anchor text the LSP validates and the resolver emits.
         let anchor = crate::ast::inline_text(&n.children);
+        let is_toc = self
+            .toc_header
+            .as_bytes()
+            .iter()
+            .zip(anchor.as_bytes())
+            .all(|(a, b)| a.eq_ignore_ascii_case(b))
+            && !self.toc_header.is_empty()
+            && self.toc_header.len() == anchor.len();
+        let mut classes = Vec::new();
+        if is_toc {
+            classes.push("toc");
+        }
+        if n.centered {
+            classes.push("centered");
+        }
+        let class = if classes.is_empty() {
+            String::new()
+        } else {
+            format!(" class=\"{}\"", classes.join(" "))
+        };
         write!(w, "<h{level}{class} id=\"")?;
         write_escaped(&anchor, w)?;
         write!(w, "\">")?;