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)?;
         }