feat: verifiable secret sharing (VSS)

[lonerapier]

Add integrity guarantees: shareholders can verify shares without revealing the secret.

Approach

• Commit to polynomial coefficients via exponentiation in a group of known order.
• Expose Dealer::commitments() returning public commitments.
• Add verify(share, commitments) → bool

Tasks

• Implement Feldman VSS commitments.
• Add verification logic
• Write example using simple modular arithmetic backend.

References

• Feldman, “A Practical Scheme for Non-Interactive Verifiable Secret Sharing” — PDF