Constant CODE_JOIN_V2 

pub const CODE_JOIN_V2: u32 = _; // 1u32

Code-join v2 (audit PA-1): the peer sends an Argon2 proof of knowledge of the join code bound to its ephemeral X25519 key + room id, instead of the cleartext bearer code. A v2 joiner omits the cleartext code entirely, so a malicious relay can no longer read it and rebind it to a forged ephemeral.