Delta Compression for RL Weight Updates

The delta compression system for RL weight updates continues to evolve, with ARCv3 significantly reducing the size of BF16 weight updates sent from the trainer to rollout machines. This enables more practical cross-region RL training by keeping rollout fleets closer to the current policy and reducing the need for massive, co-located clusters. ARCv3 achieves this by exploiting the asymmetry in BF16 weight changes, encoding mantissa-only shifts efficiently and packing rarer exponent/sign changes separately. It also employs parallel general-purpose compression algorithms for optimal payload size.