K-Group Digit-Sum Compression

While digits.length > k, split it into consecutive groups of length k, allowing a shorter final group. Replace each group by its decimal digit sum and concatenate the replacements.
Return the first string whose length is at most k.

Function
compressDigitGroups(digits: String, k: int) → String

Examples
Example 1
digits = "1111122222"
k = 3
return = "132"
Repeated group-sum passes end at 132.
Example 2
digits = "1111122222"
k = 5
return = "510"
The two length-five groups sum to 5 and 10, producing 510.

Constraints
1 <= digits.length <= 100000
2 <= k <= 100000

The core is a loop. While the string length is greater than k, walk it in chunks of k, add up the digit values in each chunk, convert each sum to a string, and join them. The last chunk can be shorter, and that's fine. The pitfall is the join. Use a list of string pieces and join once per pass, never repeated string concatenation, or you'll blow past the time budget with up to 100000 digits. Sums can be multi-digit, as example 2 shows with 510. Don't pad or truncate them. Each pass shrinks the string roughly by a factor of k when sums are small, and k is at least 2, so passes stay few. Edge case: if the length is already at most k, return the input untouched. StealthCoder is the hedge if you freeze mid-assessment, but the loop above is the whole answer.

Memorize the pattern. If you can't, run StealthCoder. The proctor sees the IDE. They don't see what's behind it.