Using Portal network protocol for history-expiry

The Finalized Chain History subnetwork is proposed as a decentralized storage layer, built on the Portal wire protocol, for finalized historical Ethereum data. It assumes nodes are Execution Layer clients that store all historical headers locally, so headers are not kept on the network. Block bodies and receipts are stored using RLP encoding, and a custom content_id function swaps the 16 least significant and 48 most significant bits of the block number, spreading content across the domain while keeping consecutive blocks close. The author chose CYCLE_BITS=16 because it simplifies implementation and because peers storing at least 1/256 (about 0.4%) of content would hold sequences of at least 256 consecutive blocks.