Add security proof materials

[joerucci]

Summary

• qualify the beta security posture and zero-knowledge scope in public docs
• add a security threat model and deterministic test vectors
• add focused crypto, security, and user-presence tests plus a security-check CI target

Validation

• go test ./...
• go vet ./...
• go test -cover ./internal/crypto ./internal/security ./internal/userpresence
• make check
• make security-check