Add more zero-knowledge proof examples

Overview

Add additional zero-knowledge proof examples to demonstrate the Decree library's capabilities and provide reference implementations for common cryptographic protocols.

Existing Examples

• Girault proof (test in tests/inscribe_tests.rs)
• Schnorr proof (test in tests/inscribe_tests.rs)

Possible Options

Simple Examples (Sigma Protocols)

• Chaum-Pedersen proof - equality of discrete logarithms (#10)
• Product proof - prove z = x * y given commitments
• Range proof - prove 0 ≤ x < 2^n using bit-by-bit approach

Complex Examples (Beyond Sigma Protocols)

• Small-factor proof - prove RSA modulus has factors above threshold (#14)
• Paillier-Blum modulus proof - prove N = pq with p ≡ q ≡ 3 (mod 4) (#14)
• MPC-in-the-head proof (e.g., Ligero-style) - demonstrates complex multi-round Fiat-Shamir
• Inner product argument (Bulletproofs-style) - shows recursive proof structure

Implementation Guidelines

• Use Ristretto255 for elliptic curve examples
• Include comprehensive test suites in example files
• Focus on clean implementations
• Demonstrate proper use of the Inscribe trait with domain separation

Start with the existing Girault and Schnorr proof tests in tests/inscribe_tests.rs and review how they use the Inscribe trait with Ristretto255. Choose and scope one example from the listed options, then add its implementation and comprehensive tests in the example files. Done means the selected protocol is demonstrated with domain separation and passing tests.