fix(html): emit heading id anchors so exported #links resolve

Exported HTML anchor links (TOC entries, [[Page#Heading]], [[#Heading]])
went nowhere because heading elements carried no `id`. Worse, the two TOC
builders slugified anchors (`#my-heading`) while the link resolver emits
the raw heading text (`#My Heading`, matching upstream vimwiki), so even
once ids existed the two halves wouldn't have agreed.

Fix — adopt vimwiki's scheme (raw heading text as the anchor) everywhere:
- render_heading emits `<hN id="<plain heading text>">`.
- build_toc_html (HTML export TOC) uses the raw, HTML-escaped title for
  both the href and the link text (was slugify).
- collect_toc_items (:VimwikiTOC buffer output) uses the raw title for the
  generated `[[#anchor]]` (was slugify). In-editor navigation slugifies
  both sides, so it still resolves.

Consistency: add canonical `nuwiki_core::ast::inline_text()` and route the
heading id, the HTML-TOC title, the buffer-TOC title, and
`diagnostics::heading_text` through it — three duplicated extractors
collapsed into one, so the anchor and its validation can't drift.

Regression test: heading_id_matches_anchor_link_href (core) asserts the
heading id and a `#anchor` link's href are identical. Updated the heading
assertions in the renderer/export tests for the new `id=` attribute.

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

crates/nuwiki-lsp/src/export.rs

@@ -18,7 +18,7 @@
 use std::collections::HashMap;
 use std::path::PathBuf;
 
-use nuwiki_core::ast::{BlockNode, DocumentNode, HeadingNode, InlineNode, LinkKind};
+use nuwiki_core::ast::{BlockNode, DocumentNode, LinkKind};
 use nuwiki_core::date::DiaryDate;
 use nuwiki_core::render::{HtmlRenderer, Renderer};
 
@@ -147,11 +147,10 @@ pub fn build_toc_html(doc: &DocumentNode) -> String {
             out.push_str("</ul>");
             depth -= 1;
         }
-        let anchor = crate::index::slugify(&h.title);
-        out.push_str(&format!(
-            "<li><a href=\"#{anchor}\">{title}</a></li>",
-            title = escape_html(&h.title)
-        ));
+        // Anchor is the raw heading text (vimwiki scheme), HTML-escaped — must
+        // match the `id` the renderer puts on the heading element.
+        let anchor = escape_html(&h.title);
+        out.push_str(&format!("<li><a href=\"#{anchor}\">{anchor}</a></li>"));
     }
     while depth > 0 {
         out.push_str("</ul>");
@@ -300,7 +299,7 @@ fn collect_headings(doc: &DocumentNode) -> Vec<HeadingProj> {
         if let BlockNode::Heading(h) = b {
             out.push(HeadingProj {
                 level: h.level,
-                title: inline_to_text(h),
+                title: nuwiki_core::ast::inline_text(&h.children),
             });
         }
     }
@@ -312,41 +311,6 @@ struct HeadingProj {
     title: String,
 }
 
-fn inline_to_text(h: &HeadingNode) -> String {
-    let mut s = String::new();
-    for n in &h.children {
-        push_inline(n, &mut s);
-    }
-    s.trim().to_string()
-}
-
-fn push_inline(n: &InlineNode, out: &mut String) {
-    match n {
-        InlineNode::Text(t) => out.push_str(&t.content),
-        InlineNode::Bold(b) => b.children.iter().for_each(|c| push_inline(c, out)),
-        InlineNode::Italic(i) => i.children.iter().for_each(|c| push_inline(c, out)),
-        InlineNode::BoldItalic(bi) => bi.children.iter().for_each(|c| push_inline(c, out)),
-        InlineNode::Strikethrough(s) => s.children.iter().for_each(|c| push_inline(c, out)),
-        InlineNode::Code(c) => out.push_str(&c.content),
-        InlineNode::Superscript(s) => s.children.iter().for_each(|c| push_inline(c, out)),
-        InlineNode::Subscript(s) => s.children.iter().for_each(|c| push_inline(c, out)),
-        InlineNode::Color(c) => c.children.iter().for_each(|c| push_inline(c, out)),
-        InlineNode::WikiLink(w) => match &w.description {
-            Some(d) => d.iter().for_each(|c| push_inline(c, out)),
-            None => {
-                if let Some(p) = &w.target.path {
-                    out.push_str(p);
-                }
-            }
-        },
-        InlineNode::ExternalLink(e) => match &e.description {
-            Some(d) => d.iter().for_each(|c| push_inline(c, out)),
-            None => out.push_str(&e.url),
-        },
-        _ => {}
-    }
-}
-
 fn escape_html(s: &str) -> String {
     let mut out = String::with_capacity(s.len());
     for ch in s.chars() {