feat(cluster): Add Phase 3 cluster routing and WAL replication

[xe-nvdk]

Phase 3.1 - Raft Foundation:

• Add hashicorp/raft-based consensus for leader election
• Implement ClusterFSM for replicated cluster state
• Add zerolog adapter for Raft logging

Phase 3.2 - Request Routing:

• Add Router with HTTP forwarding to appropriate nodes
• Support round-robin and least-connections load balancing
• Integrate router into API handlers (LineProtocol, MsgPack, Query)

Phase 3.3 - WAL Replication:

• Add entry-level streaming from writer to readers
• Implement Sender (writer-side) with fan-out to multiple readers
• Implement Receiver (reader-side) with auto-reconnect
• Add ReplicationHook to WAL for real-time entry interception

Phase 3.4 - Peer Discovery:

• Add TCP-based peer discovery protocol
• Implement JoinRequest/JoinResponse handshake
• Support seed-based cluster joining with leader redirect

Performance: Writer forwards achieve 8.8M rps (vs 11.4M direct)