Proxy Pattern

An architectural pattern for upgradeable smart contracts. A 'proxy' contract holds the storage and is the address users interact with; it forwards every call via `delegatecall` to an 'implementation' contract that holds the code. Upgrading means deploying a new implementation and pointing the proxy at it — storage is preserved. Common variants are transparent proxies (older OpenZeppelin default) and UUPS proxies (modern default). Introduces a new bug class: storage layout collisions across versions.