Fix Shamir leading coefficient generation

[KingParmenides]

Summary

• Prevent generated Shamir polynomials from using 0 as the highest-degree coefficient.
• Leave secret restoration compatible with existing shares.
• Add a regression test that forces a zero leading coefficient and verifies two shares no longer recover a 3-of-3 secret.

This applies the same defensive fix to the Python implementation linked from the Bitaps Shamir bug bounty page.

Verification

• Direct Shamir regression script with stubbed entropy: passed
• PYTHONPYCACHEPREFIX=/private/tmp/pybtc-pycache .venv/bin/python -m py_compile pybtc/functions/shamir.py tests/test_shamir_functions.py
• git diff --check

Note: full pytest tests/test_shamir_functions.py -q could not collect in this environment because the package imports compiled crypto extensions before the Shamir test module; local editable install is blocked by missing autoreconf for libsecp256k1.

Bounty payout address if accepted: 39Q34P8A7g375yqEr8buvNJkUbRgKfbKQZ