ZKsync v31 Upgrade Enables Permissionless Withdrawals

ZIP-16 proposes the v31 protocol upgrade for ZKsync, authored and sponsored by Matter Labs. It introduces Priority Mode for L1-settling chains, enabling permissionless withdrawals if the sequencer halts. The upgrade unifies Era and ZKsync OS on a shared codebase, skipping v30. A governance vote is required. OpenZeppelin audited the v31 contracts.