Function persist_received_master_key 

pub fn persist_received_master_key(
    user_id: &str,
    master_key: &[u8; 32],
) -> Result<(), CredentialError>

Persist a freshly-received 32-byte master AES key for user_id and atomically clear key_pending on the active relay profile. Called by auth::poll_key_bundle and by any other entry point (e.g. recovery-code restore) that hands the device its real master key.