fix(html): match vimwiki export output for headings, code fences, tags, anchors (#6)

Exported HTML diverged from upstream vimwiki's :VimwikiAll2HTML in ways
that break custom templates' CSS/JS and deep-links. Bring the renderer to
parity on the structural differences reported in #6:

1. Headings — restore vimwiki's structure: a wrapping
   `<div id="{hierarchical}">` (parent anchors joined by `-`),
   `class="header"`, and an in-heading `<a href="#{hierarchical}">`
   self-link. The flat `id` stays on the heading element so intra-page
   TOC / `#anchor` links keep resolving. Ancestor path is tracked while
   walking top-level headings.

2. Code fences — emit vimwiki's `<pre {raw-attrs}>` (the verbatim text
   after `{{{`, e.g. `<pre python>`) instead of
   `<pre><code class="language-X">`, so highlighters wired for vimwiki
   output apply. The fence text is now preserved verbatim through the
   lexer/AST (PreformattedNode.attrs).

3. Inline tag ids — drop the `tag-` prefix (`id="wiki"`, not
   `id="tag-wiki"`). Also fixes a real inconsistency: the LSP validated
   `[[Page#wiki]]` as resolvable while the HTML emitted `id="tag-wiki"`,
   so the exported link was broken.

4. Same-page anchors — `[[#Section]]` resolves to `index.html#Section`
   (current page filename + fragment), matching vimwiki, rather than a
   bare `#Section`.

Also ship vimwiki's stock style.css verbatim as the default stylesheet
(was a ~7-line minimal one) so exports look identical out of the box and
the `.header`/`.tag`/`.toc`/`done*` classes are styled.

Centered headings and the `<div class="toc">` Contents wrapper keep their
existing form. Tests updated across core + lsp to the new output; added
coverage for hierarchical ids, raw fence attrs, bare tag ids, and the
same-page anchor filename.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/nuwiki-core/src/syntax/vimwiki/lexer.rs

@@ -81,6 +81,9 @@ pub enum VimwikiTokenKind {
     PreformattedOpen {
         language: Option<String>,
         attrs: HashMap<String, String>,
+        /// Verbatim text after `{{{` (trailing whitespace trimmed), preserved
+        /// so the HTML renderer can reproduce vimwiki's raw `<pre …>` attrs.
+        raw: String,
     },
     PreformattedClose,
     PreformattedLine(String),
@@ -421,7 +424,14 @@ impl<'src> LexState<'src> {
         // pairs separated by spaces. Keep parsing forgiving.
         let after = trimmed[3..].trim();
         let (language, attrs) = parse_fence_attrs(after);
-        let kind = VimwikiTokenKind::PreformattedOpen { language, attrs };
+        // vimwiki keeps everything after `{{{` (minus trailing whitespace) and
+        // drops it verbatim into the `<pre>` tag, so capture it unparsed.
+        let raw = trimmed[3..].trim_end().to_string();
+        let kind = VimwikiTokenKind::PreformattedOpen {
+            language,
+            attrs,
+            raw,
+        };
         let span = Span::new(self.pos_in_line(indent), self.line_end_pos(line));
         self.push(kind, span);
         self.mode = BlockMode::Preformatted;