docs(gap): check off html_header_numbering

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

development/vimwiki-gap.md

@@ -385,7 +385,19 @@ fix site.
   not inter-page links) and the `custom_wiki2html` arg contract is an exact
   match — only the feed document shape diverges.
 - [ ] `emoji_enable` — emoji substitution.
-- [ ] `html_header_numbering` (+`_sym`) — numbered HTML headers.
+- [x] **`html_header_numbering` (+`_sym`)** — numbered HTML headers. _Fix:_
+  `HtmlConfig` (`config.rs`) gained `html_header_numbering: u8` (start level,
+  `0` = off) + `html_header_numbering_sym: String` (trailing symbol), both
+  round-tripped through `RawWiki`/defaults. A `HeadingNumberer` in
+  `nuwiki-core/src/render/html.rs` walks top-level headings in document order,
+  bumping per-depth counters and emitting a dotted prefix (`1`, `1.1`, …);
+  `render_body` numbers only document-level headers (nested ones in lists/
+  quotes stay unnumbered, matching upstream's line scan). Wired via
+  `with_header_numbering()` from `export.rs`. Tests:
+  `render_page_html_numbers_headers_when_enabled`,
+  `…_numbering_skips_headers_above_start_level`,
+  `…_no_header_numbering_by_default` (`html_export.rs`) + config round-trip +
+  defaults (`index_and_config.rs`).
 - [ ] `valid_html_tags` — allowed inline HTML tags in export.
 - [ ] `generated_links_caption`, `toc_link_format`, `markdown_link_ext`.
 - [ ] `list_ignore_newline` / `text_ignore_newline` — export newline handling.