04d2c6e548
Experiment: next_power_of_two pads prefill M up to the next pow2 (300->512), wasting matmul work. Switch to ceil-to-128 for M>128 (300->384); keep pow2 for M<=128. Byte-identical output (padding only). Baseline pp300/pp512/pp700/tg128: 30.66/40.42/40.17/2.58 t/s. Measured: 37.01/40.25/40.03/2.61 t/s. pp300 +20.7%; pp512/pp700 flat (ubatch<=512 caps M; 700=512+188, both bucketings map 188->256); tg unchanged.