改进标题渲染：按层级差异化显示（字号/加粗/间距/对齐）

src/book.rs

@@ -65,10 +65,11 @@ pub fn strip_html(input: &str) -> String {
         // Emit text before the tag
         if tag_start > pos {
             let text = decode_entities(&input[pos..tag_start]);
-            if heading_level.is_some() {
-                out.push_str("\x01");
+            if let Some(level) = heading_level {
+                out.push('\x01');
+                out.push(char::from_digit(level, 10).unwrap_or('1'));
                 out.push_str(&text);
-                out.push_str("\x02");
+                out.push('\x02');
             } else {
                 out.push_str(&text);
             }
@@ -182,7 +183,7 @@ pub struct TocEntry {
 #[derive(Debug, Clone)]
 pub struct ContentBlock {
     pub text: String,
-    pub is_heading: bool,
+    pub heading_level: u8, // 0 = body, 1-6 = h1-h6
 }
 
 #[derive(Debug, Clone)]