Single-Slot Finality

A proposed upgrade making a block economically irreversible within one slot (~12s on Ethereum) instead of the current ~15 minutes. It's hard because finalising needs a two-thirds supermajority of a very large validator set to attest every slot — an enormous networking load — so it depends on aggressive signature aggregation (BLS and beyond). The explicit constraint is delivering fast finality without shrinking or centralising the validator set.