Observable readout at peak one

Measuring the three single-site factors of $Z_B=-Y_1{Z}_2{Y}_3$ sequentially and returning their signed parity gives exact logical $Z$ readout at peak one. Similarly, $X_B=-Z_1{X}_2{Z}_3$ gives exact logical $X$ readout.

These are quantum-to-classical channels. They do not constitute the coherent decoder in Theorem thm:stream. Proposition prop:EB makes the distinction quantitative.