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-lsp/tests/index_and_config.rs

@@ -152,6 +152,28 @@ fn v1_0_single_wiki_desugars_to_one_entry() {
     assert_eq!(cfg.wikis[0].syntax, "vimwiki");
 }
 
+#[test]
+fn single_wiki_shorthand_honours_top_level_per_wiki_keys() {
+    // Regression: a single-wiki user sets per-wiki keys at the top level
+    // (alongside wiki_root); they must reach the synthesized wiki, not just
+    // file_extension/syntax. Previously toc_header_level etc. were dropped.
+    let cfg = config_from_json(json!({
+        "wiki_root": "/tmp/vimwiki",
+        "toc_header": "Table of Contents",
+        "toc_header_level": 2,
+        "links_header_level": 3,
+        "auto_export": true,
+        "html_path": "/tmp/out",
+    }));
+    assert_eq!(cfg.wikis.len(), 1);
+    let w = &cfg.wikis[0];
+    assert_eq!(w.toc_header, "Table of Contents");
+    assert_eq!(w.toc_header_level, 2);
+    assert_eq!(w.links_header_level, 3);
+    assert!(w.html.auto_export);
+    assert_eq!(w.html.html_path, PathBuf::from("/tmp/out"));
+}
+
 #[test]
 fn explicit_wikis_list_overrides_legacy_shape() {
     let cfg = config_from_json(json!({