feat(html): html_header_numbering section numbering for HTML export

Mirror vimwiki's html_header_numbering / html_header_numbering_sym:
top-level headings at or below the start level get a dotted section
number (1, 1.1, 1.2, ...) with a configurable trailing symbol. Off by
default (level 0). Nested headings in lists/quotes stay unnumbered,
matching upstream's document-level-only scan.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/nuwiki-lsp/tests/html_export.rs

@@ -41,6 +41,8 @@ fn html_config_defaults_match_vimwiki() {
     assert!(!c.auto_export);
     assert!(!c.html_filename_parameterization);
     assert!(c.exclude_files.is_empty());
+    assert_eq!(c.html_header_numbering, 0); // upstream default: numbering off
+    assert!(c.html_header_numbering_sym.is_empty());
 }
 
 #[test]
@@ -270,6 +272,73 @@ fn render_page_html_uses_metadata_date_when_set() {
     assert!(html.contains("DATE=2025-01-02"), "got: {html}");
 }
 
+#[test]
+fn render_page_html_numbers_headers_when_enabled() {
+    let ast = parse("= Intro =\n== Background ==\n== Methods ==\n= Results =\n");
+    let mut c = cfg("/tmp/x");
+    c.html.html_header_numbering = 1; // number from h1
+    c.html.html_header_numbering_sym = ".".into();
+    let html = export::render_page_html(
+        &ast,
+        None,
+        "Home",
+        DiaryDate::from_ymd(2026, 5, 11).unwrap(),
+        &c.html,
+        None,
+        -1,
+        HashMap::new(),
+    )
+    .unwrap();
+    assert!(html.contains("<h1>1. Intro</h1>"), "got: {html}");
+    assert!(html.contains("<h2>1.1. Background</h2>"), "got: {html}");
+    assert!(html.contains("<h2>1.2. Methods</h2>"), "got: {html}");
+    assert!(html.contains("<h1>2. Results</h1>"), "got: {html}");
+}
+
+#[test]
+fn render_page_html_numbering_skips_headers_above_start_level() {
+    let ast = parse("= Title =\n== Section ==\n=== Sub ===\n");
+    let mut c = cfg("/tmp/x");
+    c.html.html_header_numbering = 2; // start at h2; h1 stays unnumbered
+                                      // empty sym → number followed by a bare space
+    let html = export::render_page_html(
+        &ast,
+        None,
+        "Home",
+        DiaryDate::from_ymd(2026, 5, 11).unwrap(),
+        &c.html,
+        None,
+        -1,
+        HashMap::new(),
+    )
+    .unwrap();
+    assert!(
+        html.contains("<h1>Title</h1>"),
+        "h1 unnumbered, got: {html}"
+    );
+    assert!(html.contains("<h2>1 Section</h2>"), "got: {html}");
+    assert!(html.contains("<h3>1.1 Sub</h3>"), "got: {html}");
+}
+
+#[test]
+fn render_page_html_no_header_numbering_by_default() {
+    let ast = parse("= Intro =\n== Sub ==\n");
+    let c = cfg("/tmp/x");
+    let html = export::render_page_html(
+        &ast,
+        None,
+        "Home",
+        DiaryDate::from_ymd(2026, 5, 11).unwrap(),
+        &c.html,
+        None,
+        -1,
+        HashMap::new(),
+    )
+    .unwrap();
+    assert!(html.contains("<h1>Intro</h1>"), "got: {html}");
+    assert!(html.contains("<h2>Sub</h2>"), "got: {html}");
+}
+
 #[test]
 fn render_page_html_root_path_for_nested_pages() {
     let ast = parse("= D =\n");