Implement beacon block root contract (EIP-4788)

[mw2000]

Summary

Implement EIP-4788 which makes the parent beacon block root available to the EVM via a system contract at address 0x000F3df6D732807Ef1319fB7B8bB8522d0Beac02. Introduced in Cancun.

Specification

System Contract

• Address: 0x000F3df6D732807Ef1319fB7B8bB8522d0Beac02
• Stores beacon block roots indexed by timestamp
• Ring buffer: HISTORY_BUFFER_LENGTH = 8191
• Storage layout:
  • Slot timestamp % 8191: stores the timestamp
  • Slot (timestamp % 8191) + 8191: stores the root

System Call

At the start of each block (before user txs):

1. Call the contract with parent_beacon_block_root as calldata
2. The contract stores (parent_timestamp → parent_beacon_block_root)

User Reads

Any contract can CALL the system contract with a 32-byte timestamp to retrieve the corresponding beacon root. Returns 0 if timestamp not found in ring buffer.

Implementation Guide

1. Create lib/eevm/system_contracts/beacon_roots.ex — contract logic
2. Add parent_beacon_block_root field to Context.Block
3. Execute system call at block start — store root keyed by parent timestamp
4. Gate behind hardfork config — Cancun+
5. Tests: store root, retrieve by timestamp, ring buffer wrapping, unknown timestamp returns 0

Acceptance Criteria

• Beacon roots stored correctly in ring buffer
• CALL to contract retrieves root by timestamp
• Unknown timestamp returns empty/zero
• System call at block start works
• Gated to Cancun+ hardfork
• Tests pass

Reference

• EIP-4788: https://eips.ethereum.org/EIPS/eip-4788