Fix Shamir leading coefficient generation

[KingParmenides]

Summary

• Prevent generated Shamir polynomials from using 0 as the highest-degree coefficient.
• Preserve the existing distinct coefficient behavior and restore compatibility.
• Add a regression test that forces a zero leading coefficient and verifies two shares no longer recover a 3-of-3 secret.

This addresses the degree-collapse issue described in #45 and the Bitaps Shamir bug bounty category for implementation bugs in the presented secret sharing scheme.

Verification

• npm test (85 passing)
• git diff --check

Bounty payout address if accepted: 39Q34P8A7g375yqEr8buvNJkUbRgKfbKQZ