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/index_and_config.rs

@@ -288,6 +288,9 @@ fn defaults_carry_vimwiki_per_wiki_keys() {
     assert_eq!(cfg.html.custom_wiki2html, "");
     assert_eq!(cfg.html.custom_wiki2html_args, "");
     assert_eq!(cfg.html.base_url, "");
+    // HTML section numbering is off by default (vimwiki's `0`).
+    assert_eq!(cfg.html.html_header_numbering, 0);
+    assert_eq!(cfg.html.html_header_numbering_sym, "");
 }
 
 #[test]
@@ -319,6 +322,8 @@ fn raw_wiki_parses_every_new_key() {
             "custom_wiki2html": "~/bin/my_wiki2html.sh",
             "custom_wiki2html_args": "--flag",
             "base_url": "https://example.com/wiki/",
+            "html_header_numbering": 2,
+            "html_header_numbering_sym": ".",
         }],
     }));
     let w = &cfg.wikis[0];
@@ -350,6 +355,8 @@ fn raw_wiki_parses_every_new_key() {
     assert_eq!(w.html.custom_wiki2html, "~/bin/my_wiki2html.sh");
     assert_eq!(w.html.custom_wiki2html_args, "--flag");
     assert_eq!(w.html.base_url, "https://example.com/wiki/");
+    assert_eq!(w.html.html_header_numbering, 2);
+    assert_eq!(w.html.html_header_numbering_sym, ".");
 
     // The parsed keys drive a date-mode, Sunday-start diary calendar:
     // a weekly note is the week-start date (YYYY-MM-DD), stepping ±7 days.