feat: implement replication protocol and message codec

[bmuddha]

Problem:
Need a wire protocol for streaming transactions from primary to standby nodes. No standard exists yet for this specific use case.

Proposal:
Define message types (Heartbeat, Transaction, Block, SuperBlock, Handshake) as WebSocket frames with type prefix + bincode payload. Implement codec for encoding/decoding.

Acceptance criteria:

• Message types defined with byte identifiers
• Bincode serialization/deserialization works
• WebSocket frame wrapper implemented
• Unit tests for round-trip encoding/decoding

Reference: MIMD-0019 Phase 1

Parent epic: #935