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/diagnostics.rs

@@ -270,40 +270,11 @@ pub fn classify_outgoing(
 }
 
 /// Flatten heading inlines down to a plain string. Used by `commands::ops`
-/// to match heading text against the configured TOC/Links section name.
+/// to match heading text against the configured TOC/Links section name, and
+/// (via `nuwiki_core::ast::inline_text`) the same text the HTML renderer uses
+/// for heading `id`s — so anchors validated here match anchors emitted there.
 pub fn heading_text(inlines: &[InlineNode]) -> String {
-    let mut s = String::new();
-    push_inline_text(inlines, &mut s);
-    s.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),
-            },
-            _ => {}
-        }
-    }
+    nuwiki_core::ast::inline_text(inlines)
 }
 
 /// Resolve a `file:` / `local:` link target to an absolute path.