<think>
Okay, I need to implement the carry_add function. Let's think about what the problem says. The function takes two lists of integers, which represent counts as runs of single figures, with the most significant digit first. I need to add them and return the result in the same format. The addition should be done from the light end, which I think means starting from the least significant digit (the end of the lists since they are ordered from most to least significant). And when adding, if a pair reaches ten, carry over to the next place.

So, the steps are: iterate through the digits from the end (rightmost) of both lists, add them along with any carry from the previous step. If the sum is 10 or more, carry over the tens place to the next higher digit.

First, I need to handle the case where the lists are of different lengths. For example, if one is longer than the other, the remaining digits in the longer list should be added as is, but with any carry.

Let me outline the steps:

1. Initialize a carry variable to 0.
2. Initialize an index for both lists, starting from the end (the last element).
3. While there are elements left in either list or there's a carry:
   a. Get the current digits from left and right (if available, else 0).
   b. Sum them with the carry.
   c. The current digit is sum % 10.
   d. The new carry is sum // 10.
   e. Append the current digit to the result list.
   f. Move to the next higher digit (decrement the index).
4. After processing all digits, if there's a carry left, add it to the result.

Wait, but the result should be in the same format as the input, which is most significant first. So, when adding, the result should be built from least significant to most, but then reversed at the end?

Wait, no. Let me think again. The input lists are ordered from most significant to least. So, for example, [1, 2, 3] represents 321. When adding, we start from the least significant digit (the end of the list), so the first digits to add are 3 and 3 (if both lists are same length). Then, the next digits are 2 and 2, then 1 and 1. So, the process is to iterate from the end of both lists, adding each pair, and carrying over as needed.

But when building the result, the digits are added in the order of least significant to most. So, the first digit in the result is the least significant, and the last is the most. So, after processing, we need to reverse the result to get the correct order.

Wait, but the problem says to give the answer in the same form, which is most significant first. So, the result list should be in the same order as the input lists. So, if during addition, the digits are processed from the end (least significant) to the start (most significant), then the result list should be built in reverse order and then reversed at the end.

Wait, let's take an example. Suppose left is [1, 2], right is [3, 4]. So, the numbers are 21 and 43. Adding them: 21 + 43 = 64. The result should be [6, 4], which is most significant first. But during processing, we start with the least significant digits (2 and 3, then 1 and 4). So, the sum would be 2+3=5, then 