Pattern Finish speed training

35/35 correct; first Pattern Finish PB improved 4.914s→4.160s. Faster than full difference scanning: read only last 3 values a,b,c; p=b-a, q=c-b, d=q-p; repeat 3×: q+=d, c+=q, append c; emit array immediately, no full-sequence scan/check. 10 post-PB reps did not beat it (best 4.296s). Bottleneck: runtime variance + arithmetic.