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/html_export.rs

@@ -384,10 +384,22 @@ fn render_page_html_numbers_headers_when_enabled() {
         HashMap::new(),
     )
     .unwrap();
-    assert!(html.contains("<h1>1. Intro</h1>"), "got: {html}");
-    assert!(html.contains("<h2>1.1. Background</h2>"), "got: {html}");
-    assert!(html.contains("<h2>1.2. Methods</h2>"), "got: {html}");
-    assert!(html.contains("<h1>2. Results</h1>"), "got: {html}");
+    assert!(
+        html.contains("<h1 id=\"Intro\">1. Intro</h1>"),
+        "got: {html}"
+    );
+    assert!(
+        html.contains("<h2 id=\"Background\">1.1. Background</h2>"),
+        "got: {html}"
+    );
+    assert!(
+        html.contains("<h2 id=\"Methods\">1.2. Methods</h2>"),
+        "got: {html}"
+    );
+    assert!(
+        html.contains("<h1 id=\"Results\">2. Results</h1>"),
+        "got: {html}"
+    );
 }
 
 #[test]
@@ -407,11 +419,14 @@ fn render_page_html_numbering_skips_headers_above_start_level() {
     )
     .unwrap();
     assert!(
-        html.contains("<h1>Title</h1>"),
+        html.contains("<h1 id=\"Title\">Title</h1>"),
         "h1 unnumbered, got: {html}"
     );
-    assert!(html.contains("<h2>1 Section</h2>"), "got: {html}");
-    assert!(html.contains("<h3>1.1 Sub</h3>"), "got: {html}");
+    assert!(
+        html.contains("<h2 id=\"Section\">1 Section</h2>"),
+        "got: {html}"
+    );
+    assert!(html.contains("<h3 id=\"Sub\">1.1 Sub</h3>"), "got: {html}");
 }
 
 #[test]
@@ -428,8 +443,8 @@ fn render_page_html_no_header_numbering_by_default() {
         HashMap::new(),
     )
     .unwrap();
-    assert!(html.contains("<h1>Intro</h1>"), "got: {html}");
-    assert!(html.contains("<h2>Sub</h2>"), "got: {html}");
+    assert!(html.contains("<h1 id=\"Intro\">Intro</h1>"), "got: {html}");
+    assert!(html.contains("<h2 id=\"Sub\">Sub</h2>"), "got: {html}");
 }
 
 #[test]