Add security improvements

[rcmstark]

Summary

• Port all security fixes from Python reference implementation
• RFC 6979 deterministic nonces, Low-S normalization, public key on-curve validation, hash truncation, Montgomery ladder, Fermat's little theorem, Shamir's trick, Tonelli-Shanks, fromJacobian guard
• 73 tests across 10 separate test files matching Python structure
• Benchmark class added
• README updated with security section and benchmark numbers

Test plan

• All 73 tests passing (./gradlew test)
• Benchmark: sign 2.9ms, verify 1.8ms
• Security audit: all 9 checks pass

🤖 Generated with Claude Code