Function seal_box_encrypt 

pub fn seal_box_encrypt(
    message: &[u8],
    recipient_x25519_pk: &[u8; 32],
) -> Vec<u8> 

Encrypt a message to a recipient’s X25519 public key using a sealed box. The sender is anonymous – only the recipient can decrypt.

Reimplements crypto_box::PublicKey::seal() to avoid rand_core version mismatch (crypto_box uses rand_core 0.6, we use rand 0.9).