Lexicographically Smallest Generated String

Build the lexicographically smallest string of length n+m-1 such that for each i, the length-m window at i equals str2 if str1[i]=='T' and differs from str2 if str1[i]=='F'; the core challenge is satisfying overlapping equality constraints (propagating fixed characters) while ensuring every 'F' window has at least one mismatch, or reporting impossibility.