Fix SEP-0011 MEMO_TEXT escaping to use C-style \xNN UTF-8 byte format

lib/src/sep/0011/txrep.dart

@@ -104,9 +104,8 @@ class TxRep {
     // Preconditions — delegate to generated code.
     tx.cond.toTxRep('$prefix.cond', lines);
 
-    // Memo — handle manually because the facade uses jsonEncode/jsonDecode
-    // for legacy format compatibility, while generated code uses
-    // TxRepHelper.escapeString which emits \xNN hex escapes.
+    // Memo — handle manually so the decode side can accept both the SEP-0011
+    // \xNN format and legacy \uNNNN JSON escapes from older SDK versions.
     _encodeMemo(tx.memo, lines, '$prefix.memo');
 
     // Operations.
@@ -238,8 +237,7 @@ class TxRep {
       case XdrMemoType.MEMO_NONE:
         break;
       case XdrMemoType.MEMO_TEXT:
-        // Use JSON encoding for compatibility with the legacy format.
-        lines.add('$prefix.text: ${jsonEncode(memo.text)}');
+        lines.add('$prefix.text: ${TxRepHelper.escapeString(memo.text!)}');
         break;
       case XdrMemoType.MEMO_ID:
         memo.id!.toTxRep('$prefix.id', lines);
@@ -361,10 +359,33 @@ class TxRep {
     } else if (memoTypeStr == 'MEMO_TEXT') {
       String? textStr = TxRepHelper.getValue(map, '$prefix.text');
       if (textStr == null) throw Exception('missing $prefix.text');
-      // Decode JSON-encoded string (reverses jsonEncode() in encode).
+      // Per SEP-0011, MEMO_TEXT uses C-style \xNN UTF-8 byte escapes.
+      // For backwards compatibility, also accept legacy \uNNNN JSON escapes
+      // produced by older versions of this SDK.
       String text;
-      if (textStr.startsWith('"') && textStr.endsWith('"')) {
-        text = jsonDecode(textStr) as String;
+      if (textStr.startsWith('"') &&
+          textStr.endsWith('"') &&
+          textStr.length >= 2) {
+        // Detect an unescaped \uNNNN sequence: a \u preceded by an even
+        // number of backslashes (zero or more pairs). An odd count means the
+        // preceding backslash escapes the \u, which is literal text and
+        // should be handled by the SEP-0011 unescaper.
+        final legacyUnicode = RegExp(r'(?<!\\)(?:\\\\)*\\u[0-9a-fA-F]{4}');
+        if (legacyUnicode.hasMatch(textStr)) {
+          try {
+            text = jsonDecode(textStr) as String;
+          } on FormatException catch (e) {
+            throw Exception(
+              'invalid $prefix.text (legacy \\uNNNN form): ${e.message}',
+            );
+          }
+        } else {
+          try {
+            text = TxRepHelper.unescapeString(textStr);
+          } on FormatException catch (e) {
+            throw Exception('invalid $prefix.text: ${e.message}');
+          }
+        }
       } else {
         text = textStr;
       }

lib/src/xdr/txrep_helper.dart

@@ -127,8 +127,9 @@ class TxRepHelper {
 
   /// Escape a string for TxRep double-quoted format.
   ///
-  /// Escapes `"`, `\`, represents `\n` as `\\n`, and encodes non-ASCII bytes
-  /// (outside 0x20–0x7e) as `\\xNN`.
+  /// Escapes `"`, `\`, represents `\n`, `\r`, `\t` as their C-style short
+  /// escapes, and encodes any other non-printable or non-ASCII bytes as
+  /// `\xNN` per UTF-8 byte. Matches the stc reference and the iOS/PHP SDKs.
   static String escapeString(String s) {
     StringBuffer buf = StringBuffer();
     buf.write('"');
@@ -142,6 +143,12 @@ class TxRepHelper {
       } else if (rune == 0x0A) {
         // newline
         buf.write(r'\n');
+      } else if (rune == 0x0D) {
+        // carriage return
+        buf.write(r'\r');
+      } else if (rune == 0x09) {
+        // tab
+        buf.write(r'\t');
       } else if (rune >= 0x20 && rune <= 0x7E) {
         buf.writeCharCode(rune);
       } else {
@@ -159,8 +166,8 @@ class TxRepHelper {
 
   /// Unescape a TxRep string value.
   ///
-  /// Handles `\"`, `\\`, `\n`, and `\xNN` escape sequences. If the input is
-  /// enclosed in double quotes, they are stripped.
+  /// Handles `\"`, `\\`, `\n`, `\r`, `\t`, and `\xNN` escape sequences. If the
+  /// input is enclosed in double quotes, they are stripped.
   static String unescapeString(String s) {
     String input = s;
     if (input.startsWith('"') && input.endsWith('"') && input.length >= 2) {
@@ -193,6 +200,14 @@ class TxRepHelper {
           flushPendingBytes();
           buf.write('\n');
           i += 2;
+        } else if (next == 'r') {
+          flushPendingBytes();
+          buf.write('\r');
+          i += 2;
+        } else if (next == 't') {
+          flushPendingBytes();
+          buf.write('\t');
+          i += 2;
         } else if (next == 'x' && i + 3 < input.length) {
           String hexStr = input.substring(i + 2, i + 4);
           int? byteVal = int.tryParse(hexStr, radix: 16);