fix: validate Shamir threshold parameters

[JasonZhouPW]

Summary

• reject Shamir split requests with thresholds below 2
• reject impossible splits where total shares is less than the threshold
• add regression coverage for invalid threshold/total combinations

Why

Before this change, split_secret(0, total, secret) or split_secret(1, total, secret) could create shares that directly expose the secret or cannot be restored by restore_secret. Requests such as threshold > total also appeared to succeed but could not satisfy the requested recovery threshold.

Validation

• RED before fix: focused importlib regression expected ValueError and failed because invalid parameters were accepted
• GREEN after fix: focused importlib regression passes
• valid shamir split/restore cases OK
• PYTHONPYCACHEPREFIX=/private/tmp/pycache python3 -m py_compile pybgl/functions/shamir.py pybgl/test/shamir.py
• git diff --check

Note: python3 -m unittest pybgl.test.shamir -q still fails in this local checkout before loading tests because the native _sha3_hash extension is not built. This is unrelated to this pure-Python Shamir validation change.

Bounty context: submitted for the Bitgesell improvement/PR bounty program (#81 / #39). Preferred payout if approved: USDC/USDT on EVM-compatible rails; details can be provided privately.