EncString parse/decrypt (type 0 and type 2: AES-256-CBC + HMAC-SHA256
over iv||ciphertext, PKCS#7), the machine-credential format
(0.<uuid>.<secret>:<key>), and the HKDF shareable-key derivation that
unwraps the organization key. Stdlib only. Test vectors come from the
public Bitwarden SDK test suite, so the construction matches the official
clients exactly.
