-----Description----- 
This task requires writing a Lean 4 method that computes the sum of the digits of a non-negative integer. The method should process each digit of the input number and return the total sum. The output is guaranteed to be a non-negative natural number.

-----Input-----
The input consists of:
n: A non-negative integer.

-----Output-----
The output is a natural number:
Returns the sum of the digits of the input integer.

-----Note-----
The input is assumed to be a valid non-negative integer.