DerivationPath: support 'h' in Display output for hardened components

Aligns with ChildNumber’s format and improves consistency when debugging or comparing derivation paths.

Resolves: rust-bitcoin#4618

Author: vicjuma

bitcoin/src/bip32.rs

@@ -482,11 +482,19 @@ impl fmt::Display for DerivationPath {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         let mut iter = self.0.iter();
         if let Some(first_element) = iter.next() {
-            write!(f, "{}", first_element)?;
+            if f.alternate() {
+                write!(f, "{:#}", first_element)?;
+            } else {
+                write!(f, "{}", first_element)?;
+            }
         }
         for cn in iter {
             f.write_str("/")?;
-            write!(f, "{}", cn)?;
+            if f.alternate() {
+                write!(f, "{:#}", cn)?;
+            } else {
+                write!(f, "{}", cn)?;
+            }
         }
         Ok(())
     }
@@ -1108,6 +1116,13 @@ mod tests {
         }
     }
 
+    #[test]
+    fn test_derivation_path_display() {
+        let path = DerivationPath::from_str("m/84'/0'/0'/0/0").unwrap();
+        assert_eq!(format!("{}", path), "84'/0'/0'/0/0");
+        assert_eq!(format!("{:#}", path), "84h/0h/0h/0/0");
+    }
+
     #[test]
     fn parse_derivation_path_out_of_range() {
         let invalid_path = "2147483648";