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/tests/html_renderer.rs

@@ -253,6 +253,33 @@ fn heading_id_matches_anchor_link_href() {
     );
 }
 
+#[test]
+fn toc_header_heading_wrapped_in_div_toc() {
+    // The TOC section heading gets a `<div class="toc">` wrapper (vimwiki
+    // scheme) so `.toc` stylesheet rules apply. The class is on the div, not
+    // the heading.
+    let doc = VimwikiSyntax::new().parse("== Contents ==\n");
+    let out = HtmlRenderer::new()
+        .with_toc_header("Contents")
+        .render_to_string(&doc)
+        .unwrap();
+    assert!(
+        out.contains("<div class=\"toc\"><h2 id=\"Contents\">Contents</h2></div>"),
+        "got: {out}"
+    );
+}
+
+#[test]
+fn non_toc_heading_not_wrapped() {
+    let doc = VimwikiSyntax::new().parse("== Intro ==\n");
+    let out = HtmlRenderer::new()
+        .with_toc_header("Contents")
+        .render_to_string(&doc)
+        .unwrap();
+    assert!(out.contains("<h2 id=\"Intro\">Intro</h2>"), "got: {out}");
+    assert!(!out.contains("class=\"toc\""), "got: {out}");
+}
+
 #[test]
 fn interwiki_numbered_link() {
     let out = render("[[wiki1:Page]]\n");