Lexicographically Smallest String After Substring Operation

Given a lowercase string and the ability to perform exactly one operation that decrements every character in any chosen non-empty substring by one (with 'a' wrapping to 'z'), choose the substring to produce the lexicographically smallest resulting string. The challenge is to pick the optimal contiguous segment under that wraparound rule to minimize lexicographic order for strings up to 3e5 length.