Split crypto.rs into crypto/ module directory
Break the monolithic crypto.rs (253 lines) into focused submodules:
crypto/mod.rs (types, constants, re-exports), content.rs (AES-256-GCM),
key_wrapping.rs (x25519-hkdf-a256kw), keyring_wrapping.rs (symmetric
AES-KW for group keys). Test file moved into module directory.