Add pure Rust AES-GCM authenticated encryption in the crypto crate: - AES block cipher (FIPS 197): key expansion and encrypt for 128/256-bit keys - GHASH: constant-time GF(2^128) multiplication using bit masking - GCM encrypt/decrypt with 96-bit nonce, 128-bit authentication tag, and AAD - Constant-time tag comparison for secure decryption - 25 tests including NIST SP 800-38D test vectors (cases 1-4, 13-16), tag tamper detection, and encrypt/decrypt round-trips Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>