Function encode_session_env 

pub fn encode_session_env(sid_hex: &str, ephemeral_private: &[u8; 32]) -> String

Encode a JOY_SESSION env var value from the session id (hex) and the 32-byte ephemeral private key. Layout of the decoded payload: [sid_raw 12 bytes][ephemeral_private 32 bytes], base64-encoded.