Upgrade 18 Launches Custom Gas Token v2 and Kona Proofs

Protocol Upgrade U18 advances OP Stack with Custom Gas Token v2 enabling non-ETH gas assets, refines dispute games via clone-with-immutable-arguments reducing operational costs below EIP-7825 gas limit, and adds Rust-based Kona alongside Cannon proofs for fault proof redundancy. Contracts at op-contracts/v6.0.0-rc.2 finalize to v6.0.0 post-governance. Verified prestates: op-program v1.9.0-rc.1 and Kona v1.2.7. Multichain rollout includes OP Mainnet and others.