fix(toc): honour toc_header/level in single-wiki config + div.toc on export

Two TOC bugs:

1. :NuwikiTOC ignored toc_header / toc_header_level (always "= Contents ="
   at level 1). In the single-wiki shorthand the server only carried
   file_extension/syntax from the top-level options into the synthesized
   wiki; every other per-wiki key (toc_header, toc_header_level,
   links_header, html_path, auto_export, …) was dropped. Add
   single_wiki_from_value(), which re-parses the top-level object as a full
   RawWiki (injecting root from wiki_root), so all per-wiki keys set at the
   top level flow through. Wired into from_init_params + apply_change.

2. HTML export put class="toc" on the <hN> element, but upstream vimwiki
   (and its stylesheet) wraps the TOC heading in <div class="toc">…</div>.
   render_heading now emits the div wrapper, matching upstream so .toc CSS
   applies. With (1) fixed, detection also works for a custom toc_header.

Tests: single_wiki_shorthand_honours_top_level_per_wiki_keys (config),
toc_header_heading_wrapped_in_div_toc + non_toc_heading_not_wrapped
(renderer). Full suite 572 passed; clippy clean; config-parity goldens
match.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -60,8 +60,9 @@ pub struct HtmlRenderer {
     /// 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.
+    /// section. A heading matching this value (case-insensitive) is wrapped
+    /// in `<div class="toc">…</div>` on export, matching upstream so vimwiki's
+    /// stylesheet applies.
     toc_header: String,
     /// vimwiki `valid_html_tags`: inline HTML tag names passed through to
     /// export verbatim (instead of escaping `<`/`>`). Empty = escape all.
@@ -327,25 +328,18 @@ impl HtmlRenderer {
     fn render_heading(&self, n: &HeadingNode, number: &str, w: &mut dyn Write) -> io::Result<()> {
         let level = n.level.clamp(1, 6);
         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();
+        // The TOC section heading (vimwiki `toc_header`) is wrapped in a
+        // `<div class="toc">`, matching upstream's HTML so vimwiki's stylesheet
+        // (`.toc { … }`) applies. The class goes on the wrapping div, not the
+        // heading element.
+        let is_toc = !self.toc_header.is_empty() && anchor.eq_ignore_ascii_case(&self.toc_header);
         if is_toc {
-            classes.push("toc");
+            w.write_all(b"<div class=\"toc\">")?;
         }
-        if n.centered {
-            classes.push("centered");
-        }
-        let class = if classes.is_empty() {
-            String::new()
+        let class = if n.centered {
+            " class=\"centered\""
         } else {
-            format!(" class=\"{}\"", classes.join(" "))
+            ""
         };
         write!(w, "<h{level}{class} id=\"")?;
         write_escaped(&anchor, w)?;
@@ -354,7 +348,11 @@ impl HtmlRenderer {
             write_escaped(number, w)?;
         }
         self.render_inlines(&n.children, w)?;
-        writeln!(w, "</h{level}>")
+        write!(w, "</h{level}>")?;
+        if is_toc {
+            w.write_all(b"</div>")?;
+        }
+        writeln!(w)
     }
 
     fn render_paragraph(&self, n: &ParagraphNode, w: &mut dyn Write) -> io::Result<()> {