Implement Block Header Structure with Consensus Verification Support

[kingofthebongo2008]

Description

The Architecture.md specifies the need for light sync to verify canonical chain without full blocks. While tests demonstrate block header encoding/decoding patterns, there is no formal BlockHeader structure or consensus verification utilities. Per Issue #2, this is essential for validating cross-chain messages using RLP-encoded state proofs.

Required for:

• Light client header chain synchronization
• Parent hash chain verification
• Block header validation for each chain (Ethereum PoS, Polygon, BSC PoSA)

Acceptance Criteria

• Create BlockHeader struct with all 15+ header fields (parentHash, uncleHash, coinbase, stateRoot, etc.)
• Implement encodeBlockHeader(RlpEncoder&, const BlockHeader&) free function
• Implement decodeBlockHeader(RlpDecoder&, BlockHeader&) free function
• Implement computeBlockHash(const BlockHeader&) -> Hash256 utility
• Implement verifyParentChain(span<const BlockHeader>) for chain consistency
• Add chain-specific verification hooks (extensible for PoS, PoSA)
• Add unit tests with real Ethereum mainnet block headers
• Place in rlp_ethereum.hpp or new rlp_ethereum_blocks.hpp

Priority

High

Related Requirements

• CLAUDE.md: Const-correctness - All parameter structs passed by const&
• CLAUDE.md: Named initialization - Structs designed for designated initializers
• Issue EVM Bridging Messaging System #2: Bridge Message Processing - Validates cross-chain messages using RLP-encoded state proofs