Third per-consumer step of stage 2b, the read-modify-write one. m_acc_src is
now a registered read (m_acc_rd) whose address leads by pm_vo (m_oidx+1 when
consuming) so the value lands exactly on the pm_vo cycle. Same-index RMW on
bank_t is read-old: the running-t_hat source was written by the previous term
and is long settled; read addr (m_oidx+1) leads write addr (m_oidx).
11/11 KAT PASS incl. K=3/4 j>0 paths, byte-exact.