[Rust] Validator client — Phase 2 core implementation

[fatdinhero]

Implement the AgentsProtocol validator client in Rust.

Stack: tokio + libp2p + serde

Scope:

• Claim submission and parsing
• S_con computation against versioned corpus
• Ψ (Psi) non-collusion statistic
• GHOSTDAG block weight calculation
• P2P networking layer

Reference: docs/architecture.md, docs/math.md
Difficulty: Advanced
Phase: 2