contracts-bedrock: audit prep for OPCMMigrator

### Audit prep for the the interop migration path and lockbox changes

`ETHLockbox` becomes the pause identifier and is deployed on every chain through `upgrade()`, `SystemConfig`, `AnchorStateRegistry` and `DelayedWETH` resolve pause and guardian through it, and `OPContractsManagerMigrator` and its validator are hardened. All of it needs an external audit before the upgrade ships.

- Define dates.
- Define the scope as the diff of every changed contract.
- Prepare the briefing: architecture before and after, invariants, known issues and their status, test coverage, out of scope.
- Triage the findings and agree on a status for each.