Documents: the field highlighter reads front matter as front matter

Without the front matter construct, an open front matter block parsed
as a rule and a setext heading, so its keys drew bright and bold. The
highlighter now parses as the file does.

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

src/documents/markdown.rs

@@ -83,7 +83,7 @@ pub(crate) fn walk<'a>(node: &'a mdast::Node, out: &mut Vec<&'a mdast::Node>) {
 
 /// The marks over `text`, with the syntax writing each.
 pub fn marks(text: &str) -> Vec<(Range<usize>, Mark)> {
-    let root = tree(text);
+    let root = file_tree(text);
     let mut nodes = Vec::new();
     walk(&root, &mut nodes);
     let mut out = Vec::new();