Expression Stack speed training

21/21 correct; first Expression Stack PB 6.606s→4.254s. Reuse fixed shell after confirming: read only five number slots a,b,c,d,e from [a,b,+,c,*,d,-,e,max], compute r=(a+b)*c-d; if r>=10 emit r immediately, else emit max(r,e), as a plain string. No stack replay/second check. Bottleneck: connector/runtime variance.