Rearrange K Substrings to Form Target String

Given anagram strings s and t and an integer k (with |s| divisible by k), determine whether you can split s into k equal-length contiguous substrings and permute those substrings to form t. The core challenge is to check whether the multisets of length-|s|/k substrings obtained from s and t are identical (efficiently, since |s| can be up to 2e5).