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-core/src/ast/inline.rs

@@ -66,6 +66,46 @@ impl InlineNode {
     }
 }
 
+/// Concatenate the plain-text content of a run of inlines, descending into
+/// formatting containers. Links/external links use their description, falling
+/// back to the target path / URL. This is the canonical heading/anchor text —
+/// the HTML renderer uses it for heading `id`s and the LSP for anchor matching,
+/// so the two never drift.
+pub fn inline_text(inlines: &[InlineNode]) -> String {
+    let mut out = String::new();
+    push_inline_text(inlines, &mut out);
+    out.trim().to_string()
+}
+
+fn push_inline_text(inlines: &[InlineNode], out: &mut String) {
+    for n in inlines {
+        match n {
+            InlineNode::Text(t) => out.push_str(&t.content),
+            InlineNode::Bold(b) => push_inline_text(&b.children, out),
+            InlineNode::Italic(i) => push_inline_text(&i.children, out),
+            InlineNode::BoldItalic(bi) => push_inline_text(&bi.children, out),
+            InlineNode::Strikethrough(s) => push_inline_text(&s.children, out),
+            InlineNode::Code(c) => out.push_str(&c.content),
+            InlineNode::Superscript(s) => push_inline_text(&s.children, out),
+            InlineNode::Subscript(s) => push_inline_text(&s.children, out),
+            InlineNode::Color(c) => push_inline_text(&c.children, out),
+            InlineNode::WikiLink(w) => match &w.description {
+                Some(d) => push_inline_text(d, out),
+                None => {
+                    if let Some(p) = &w.target.path {
+                        out.push_str(p);
+                    }
+                }
+            },
+            InlineNode::ExternalLink(e) => match &e.description {
+                Some(d) => push_inline_text(d, out),
+                None => out.push_str(&e.url),
+            },
+            _ => {}
+        }
+    }
+}
+
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct TextNode {
     pub span: Span,

crates/nuwiki-core/src/ast/mod.rs

@@ -16,8 +16,9 @@ pub use block::{
     TagScope,
 };
 pub use inline::{
-    BoldItalicNode, BoldNode, CodeNode, ColorNode, InlineNode, ItalicNode, Keyword, KeywordNode,
-    MathInlineNode, SoftBreakNode, StrikethroughNode, SubscriptNode, SuperscriptNode, TextNode,
+    inline_text, BoldItalicNode, BoldNode, CodeNode, ColorNode, InlineNode, ItalicNode, Keyword,
+    KeywordNode, MathInlineNode, SoftBreakNode, StrikethroughNode, SubscriptNode, SuperscriptNode,
+    TextNode,
 };
 pub use link::{
     ExternalLinkNode, LinkKind, LinkTarget, RawUrlNode, TransclusionNode, WikiLinkNode,

crates/nuwiki-core/src/render/html.rs

@@ -319,7 +319,13 @@ impl HtmlRenderer {
         } else {
             ""
         };
-        write!(w, "<h{level}{class}>")?;
+        // `id` is the heading's plain text (vimwiki's anchor scheme), so
+        // `[[Page#Heading]]` and TOC `#Heading` links land here. Matches the
+        // anchor text the LSP validates and the resolver emits.
+        let anchor = crate::ast::inline_text(&n.children);
+        write!(w, "<h{level}{class} id=\"")?;
+        write_escaped(&anchor, w)?;
+        write!(w, "\">")?;
         if !number.is_empty() {
             write_escaped(number, w)?;
         }

crates/nuwiki-core/tests/html_renderer.rs

@@ -63,14 +63,14 @@ fn heading_levels() {
     for level in 1..=6 {
         let bars = "=".repeat(level);
         let out = render(&format!("{bars} Title {bars}\n"));
-        assert!(out.starts_with(&format!("<h{level}>Title</h{level}>")));
+        assert!(out.starts_with(&format!("<h{level} id=\"Title\">Title</h{level}>")));
     }
 }
 
 #[test]
 fn centered_heading_gets_centered_class() {
     let out = render("  = T =\n");
-    assert!(out.contains("<h1 class=\"centered\">T</h1>"));
+    assert!(out.contains("<h1 class=\"centered\" id=\"T\">T</h1>"));
 }
 
 #[test]
@@ -237,6 +237,22 @@ fn anchor_only_link() {
     assert!(out.contains("<a href=\"#Section\">"));
 }
 
+#[test]
+fn heading_id_matches_anchor_link_href() {
+    // The heading carries an `id` equal to its plain text, so a `#anchor`
+    // link (TOC or cross-reference) actually lands on it. Regression guard
+    // for "exported HTML anchor links don't work".
+    let out = render("= My Section =\n\n[[#My Section]]\n");
+    assert!(
+        out.contains("<h1 id=\"My Section\">My Section</h1>"),
+        "heading id: {out}"
+    );
+    assert!(
+        out.contains("<a href=\"#My Section\">"),
+        "anchor href: {out}"
+    );
+}
+
 #[test]
 fn interwiki_numbered_link() {
     let out = render("[[wiki1:Page]]\n");
@@ -299,7 +315,7 @@ fn template_substitutes_content_and_title() {
     );
     let out = renderer.render_to_string(&doc).unwrap();
     assert!(out.contains("<title>My Page</title>"));
-    assert!(out.contains("<body><h1>Hello</h1>"));
+    assert!(out.contains("<body><h1 id=\"Hello\">Hello</h1>"));
 }
 
 #[test]
@@ -308,7 +324,7 @@ fn template_with_missing_title_substitutes_empty_string() {
     let renderer = HtmlRenderer::new().with_template("<title>{{title}}</title>{{content}}");
     let out = renderer.render_to_string(&doc).unwrap();
     assert!(out.contains("<title></title>"));
-    assert!(out.contains("<h1>Heading</h1>"));
+    assert!(out.contains("<h1 id=\"Heading\">Heading</h1>"));
 }
 
 #[test]
@@ -323,7 +339,10 @@ fn template_substitutes_vimwiki_percent_placeholders() {
         .with_var("root_path", "../");
     let out = renderer.render_to_string(&doc).unwrap();
     assert!(out.contains("<title>My Page</title>"), "title: {out}");
-    assert!(out.contains("<body><h1>Hello</h1>"), "content: {out}");
+    assert!(
+        out.contains("<body><h1 id=\"Hello\">Hello</h1>"),
+        "content: {out}"
+    );
     assert!(out.contains("href=\"../style.css\""), "root_path: {out}");
     assert!(!out.contains('%'), "no placeholder left: {out}");
 }
@@ -415,7 +434,7 @@ fn x() {}
     let out = HtmlRenderer::new().render_to_string(&doc).unwrap();
     // A few sanity checks; the full output is exercised piece-by-piece above.
     for needle in [
-        "<h1>Heading</h1>",
+        "<h1 id=\"Heading\">Heading</h1>",
         "<strong>bold</strong>",
         "<em>italic</em>",
         "<code>code</code>",