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/tests/lexer.rs

@@ -179,7 +179,9 @@ fn preformatted_block_parses_language() {
 #[test]
 fn preformatted_block_parses_attrs() {
     let lexed = lex("{{{rust class=\"hl\" key=val\nx\n}}}\n");
-    let PreformattedOpen { language, attrs } = &lexed[0] else {
+    let PreformattedOpen {
+        language, attrs, raw, ..
+    } = &lexed[0] else {
         panic!("expected PreformattedOpen");
     };
     assert_eq!(language.as_deref(), Some("rust"));
@@ -187,6 +189,8 @@ fn preformatted_block_parses_attrs() {
     expected.insert("class".into(), "hl".into());
     expected.insert("key".into(), "val".into());
     assert_eq!(attrs, &expected);
+    // The verbatim fence text is preserved for the HTML renderer.
+    assert_eq!(raw, "rust class=\"hl\" key=val");
 }
 
 // ===== Math block =====