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/tests/link_health.rs

@@ -492,7 +492,9 @@ fn toc_edit_inserts_at_top_when_no_existing_toc() {
     // Insertion at position 0,0 → zero-width range.
     assert_eq!(te.range.start, te.range.end);
     assert!(te.new_text.starts_with("= Contents =\n"));
-    assert!(te.new_text.contains("[[#one|One]]"));
+    // Anchor is the raw heading text (vimwiki scheme), matching the heading id
+    // the HTML renderer emits on export.
+    assert!(te.new_text.contains("[[#One|One]]"));
 }
 
 #[test]
@@ -504,7 +506,7 @@ fn toc_edit_replaces_existing_toc() {
     let changes = edit.changes.expect("changes map");
     let edits = &changes[&uri];
     let te = &edits[0];
-    assert!(te.new_text.contains("[[#real|Real]]"));
+    assert!(te.new_text.contains("[[#Real|Real]]"));
     assert!(!te.new_text.contains("Stale"));
     // The replacement range must start at line 0.
     assert_eq!(te.range.start.line, 0);
@@ -536,7 +538,7 @@ fn toc_rebuild_edit_only_acts_when_toc_already_present() {
     let edit =
         ops::toc_rebuild_edit(with, &ast, &uri, true, "Contents", 1, 0, 0).expect("rebuild edit");
     let te = &edit.changes.unwrap()[&uri][0];
-    assert!(te.new_text.contains("[[#real|Real]]"));
+    assert!(te.new_text.contains("[[#Real|Real]]"));
     assert!(!te.new_text.contains("Stale"));
 }