Add Digits - Loop Approach

The direct approach: while $n \geq 10$, extract each digit using $n \bmod 10$, add them up, and repeat with the sum.

For $n = 38$: extract $8$ and $3$, sum is $11$. For $n = 11$: extract $1$ and $1$, sum is $2$. Now $n < 10$, so return $2$.

This works but takes multiple iterations. There's a faster way using modulo arithmetic.