fix(jsbn2): reject non-positive values in primality checks

[Kr0emer]

Summary

This PR fixes BigInteger primality checks for non-positive inputs.

isProbablePrime previously operated on abs(this) and could return true
for negative values (e.g. -7, -97). Since primes are defined only for
positive integers greater than 1, this is incorrect behavior.

Changes

• ext/jsbn2.js
  • bnIsProbablePrime(t): now returns false when this.signum() <= 0
  • bnpMillerRabin(t): adds the same defensive guard for non-positive input
• test/qunit-do-crypto.html
  • Added regression test: negative values must return false
  • Added control test: positive prime/composite behavior remains unchanged

Why

This ensures primality checks are mathematically correct and avoids accepting
invalid negative parameters in callers that rely on isProbablePrime.

Validation

• Manual check after patch:
  • -2, -7, -13, -97 -> false
  • 7 -> true, 4 -> false
• Ran test suite:
  • npm test passes