S12 READMEs: arms A and B2 stopped after segment 2

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auto ckpt ckpt_440k.pt

S12 READMEs: arm B2 (forked from A at 420k), segment-2 selection axis, arm B stopped after segment 1

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README for building pair 1 (s12A arcmix pool, s12B arcmix+edu): why they exist, setup