Add pure Rust ChaCha20-Poly1305 authenticated encryption in the crypto crate: - ChaCha20 stream cipher: quarter round, block function (20 rounds), counter-mode encryption with 256-bit key, 96-bit nonce, 32-bit counter - Poly1305 one-time MAC: GF(2^130-5) using 5 limbs of 26 bits, clamped r key, constant-time final reduction - AEAD construction: Poly1305 key generation from ChaCha20 block 0, encrypt-then-MAC with RFC 8439 padding and length encoding - Constant-time tag comparison for secure decryption - 13 tests: RFC 8439 test vectors (§2.1.1, §2.3.2, §2.4.2, §2.5.2, §2.6.2, §2.8.2), tag tamper detection, and round-trip tests Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>