testnet(amaru): set epoch_length=256 so the stability window fits within an epoch
stability_window=3k/f=150 and randomness_stabilization_window=4k/f=200 must be smaller than the epoch; the previous epoch_length=120 made the rewards stake snapshot never stabilize in-epoch, so the relay died with RewardsSummaryNotReady at the first boundary. 256 clears both windows.