3972. Valid Subarrays With Matching Sum Digits II

Since all elements are positive, prefix sums are strictly increasing.

A sum has first digit x iff it belongs to one of the ranges [x * 10p, (x + 1) * 10p - 1].

Also require the sum to be congruent to x modulo 10, then count prefix-sum pairs whose difference lies in each valid range and residue class.