fix: #48 markdown.rs 处理 FrontMatter 和 FootnoteDefinition 节点

FrontMatter → CodeBlock(language=yaml)
FootnoteDefinition → Paragraph

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/zx_document_core/src/markdown.rs

@@ -158,6 +158,25 @@ fn collect_blocks<'a>(node: &'a AstNode<'a>, blocks: &mut Vec<DocumentBlock>) {
                     })
                 }
             }
+            NodeValue::FrontMatter(content) => {
+                if content.trim().is_empty() {
+                    None
+                } else {
+                    Some(DocumentBlock::CodeBlock {
+                        id: block_id(),
+                        language: Some("yaml".into()),
+                        code: content.clone(),
+                    })
+                }
+            }
+            NodeValue::FootnoteDefinition(_) => {
+                let text = collect_text(child);
+                if text.is_empty() {
+                    None
+                } else {
+                    Some(DocumentBlock::Paragraph { id: block_id(), text })
+                }
+            }
             NodeValue::Document
             | NodeValue::Item(_)
             | NodeValue::TableCell