documents: shortcodes read per DOM text node as GitHub reads them; HTML5's p closers end a p; a TOC press follows editor anchors

- The first `:name:`-shaped run in a text node decides for the whole node:
  blank text or a space before it turns every shortcode, else none. Filtered
  tags are text and do not split the node. Probed on GitHub's markdown API.
- `fieldset`, `menu` and the rest of HTML5's list end an open `p`.
- `CodeEditor` carries owner anchors through each edit; the document anchors
  every heading it reads, and a TOC press goes where its anchor went.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/documents/document/mod.rs

@@ -316,6 +316,9 @@ impl MarkdownDocument {
         let old = self.parsed.blocks.len();
         self.list
             .splice(head..old - tail, read.blocks.len() - head - tail);
+        let headings: Vec<usize> = read.headings.iter().map(|heading| heading.at).collect();
+        self.editor
+            .update(cx, |editor, _| editor.set_anchors(headings));
         self.parsed = Arc::new(read);
         cx.emit(DocumentEvent::Parsed);
         cx.notify();