docs: private final void _appendCharacterEscape(char ch, int escCode)
          throws IOException, JsonGenerationException
      {
          if (escCode >= 0) { // \\N (2 char)
              if ((_outputTail + 2) > _outputEnd) {
                  _flushBuffer();
              }
              _outputBuffer[_outputTail++] = '\\';
              _outputBuffer[_outputTail++] = (char) escCode;
              return;
          }
          if (escCode != CharacterEscapes.ESCAPE_CUSTOM) { // std, \\uXXXX
              if ((_outputTail + 2) > _outputEnd) {
                  _flushBuffer();
              }
              int ptr = _outputTail;
              char[] buf = _outputBuffer;
              buf[ptr++] = '\\';
              buf[ptr++] = 'u';
              // We know it's a control char, so only the last 2 chars are non-0
              if (ch > 0xFF) { // beyond 8 bytes
                  int hi = (ch >> 8) & 0xFF;
                  buf[ptr++] = HEX_CHARS[hi >> 4];
                  buf[ptr++] = HEX_CHARS[hi & 0xF];
                  ch &= 0xFF;
              } else {
                  buf[ptr++] = '0';
                  buf[ptr++] = '0';
              }
              buf[ptr++] = HEX_CHARS[ch >> 4];
              buf[ptr] = HEX_CHARS[ch & 0xF];
              _outputTail = ptr;
              return;
          }
      }