Unique Substrings in Wraparound String

Count the number of distinct non-empty substrings of s that appear in the infinite wraparound alphabet (i.e., substrings where each character is the next letter modulo 26). The key observation is to track, for each ending character, the longest contiguous wraparound run ending there and sum those lengths to get the number of unique substrings (O(n) time, n up to 1e5).