Decoder C: two retained qubits

Retain the first two occupied rails. For each later occupied rail $a$, apply ${\mathrm{CX}}_{b_1\to a}$, ${\mathrm{CZ}}_{b_2,a}$, and $H_a$; measure and consume $a$, applying ${X_{b_1}}^z$. At the horizon, if two rails remain, apply ${\mathrm{CX}}_{b_1\to b_2}$, $H_{b_2}$, measure and consume $b_2$, and apply ${Z_{b_1}}^z$. Return the remaining rail. One or zero retained rails are handled by direct return or $I/2$ preparation. The peak is at most $q_A+2$.