8adda8448c
Experiment: decode runs M=1; opening+joining an OpenMP parallel region per row (A-prep and C-collect loops) is pure fork/join overhead. Guard both pragmas with if(M>=8) so decode runs serially while prefill (M>=300) stays parallel. Byte-identical output. tg128 2.57 -> 2.73-2.80 (+6-9%), tg256 2.65; pp unchanged (35.8/41.6/40.6 for 300/512/700). Zero risk (same math, threading policy only).