staking/v0 package

Package staking is a simplified, accounting-only port of Synthetix StakingRewards to gno.land. Users stake a fungible amount; rewards accrue every block, proportional to each staker's share of the total staked, at a fixed rewardPerBlock. There is no real coin transfer — balances and rewards are plain uint64 accounting (a demonstration of the classic "reward-per-token" accumulator pattern).