docs: add comprehensive threat model specification (closes #7)

[rupesh-kumar-sah]

Summary

This PR resolves issue #7 by adding to formally document the security threat model for the Corridor protocol.

Coverage

• Assets: User KYC privacy, credential integrity, double-spend resistance via single-use nullifiers, policy enforcement soundness, and storage TTL rent continuity.
• Adversary Models:
  • Dishonest relayer ()
  • Malicious holder ()
  • Compromised issuer key ()
  • Compromised auditor key ()
  • Verifier soundness bug ()
• Per-Layer Attacks & Mitigations: Protocol/Circuit layer (Noir/UltraHonk), Soroban contract layer, and network/relayer layer.
• Out of Scope: Explicit pre-mainnet assumptions and out-of-scope boundaries.

Closes #7

[vercel]

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