feat: publicly verifiable secret sharing (PVSS)

[lonerapier]

Enable anyone (not just participants) to verify the validity of shares via public commitments.

Approach

• Build on the Feldman VSS design.
• Use EC-based commitments (Ristretto255).
• Define PvssDealer and PvssVerifier traits.
• Generate test vectors compatible with curve25519-dalek.

Tasks

• Implement EC commitment scheme.
• Add serialization for commitments.
• Provide public verification functions.
• Add CLI support: ssskit verify-share --commitments commitments.json

References

• Stadler, “Publicly Verifiable Secret Sharing” — PDF