fix(jsbn2): handle negative exponents in BigInteger.modPow

[Kr0emer]

Summary

This PR fixes BigInteger.modPow() for negative exponents in jsbn2.

Previously, negative exponents were processed through two's-complement bit scanning, which produced silent incorrect results (e.g. a.modPow(-1, m) often returned 1).

Root Cause

bnModPow used e.bitLength() and exponent bit scanning without checking exponent sign, so negative values were interpreted as invalid positive bit patterns.

Fix

In ext/jsbn2.js, bnModPow(e, m) now handles negative exponents explicitly:

• if e < 0, compute:
  • this.modInverse(m).modPow(e.negate(), m)

This preserves existing behavior for non-negative exponents and returns mathematically correct values for negative exponents.

Tests

Added regression tests in test/qunit-do-crypto.html:

• BigInteger.modPow with exponent -1
• BigInteger.modPow with exponent -k

Both compare against the mathematically correct construction using modInverse.

Verification

• Reproduced original issue before patch: multiple negative-exponent cases mismatched expected values.
• After patch: all tested cases matched expected results.
• Ran npm test in npm/: all tests passed.