Implement full transaction serialization with RLP #16

Description

Complete the transaction serialization implementation in SignerMiddleware.

Location

• File: src/middleware/signer.zig
• Lines: 152, 167

Current Status

Currently stubbed to return minimal signature data (65 bytes: r, s, v).

Tasks

• Implement full RLP encoding for legacy transactions
• Implement full RLP encoding for EIP-2930 transactions
• Implement full RLP encoding for EIP-1559 transactions
• Implement full RLP encoding for EIP-4844 transactions
• Implement full RLP encoding for EIP-7702 transactions
• Include signature in serialized output
• Add comprehensive tests

Priority

Medium - Needed for actual transaction broadcasting