feat: threshold changeable secret resharing (TCSS)

[lonerapier]

Enable changing (k, n) parameters or participant set without reconstructing the secret.

Approach

• Implement reshare(old_shares, new_n, new_k) protocol.
• Add optional verification (based on commitments).
• Handle resharing with partial sets and secure randomness.
• Integrate with ssskit-cli.

Tasks

• Add new module resharing.
• Implement offline resharing algorithm.
• Add proofs-of-consistency and tests.
• CLI: ssskit reshare --input old_shares/ --new-n 7 --new-k 4

References

• Alin Tomescu, “How to Reshare a Secret” — Blog
• Meng et al., “Threshold Changeable Secret Sharing with Secure Secret Reconstruction” — ScienceDirect
• “Resharing Shamir Secret Shares to Change the Threshold” — Conduition.io