Function encode_secret_key_list 

pub fn encode_secret_key_list<'a, I>(keys: I) -> Vec<u8> 
where
    I: IntoIterator<Item = &'a [u8]>,

Serialize a list of raw secret keys into the wire format read back by decode_secret_key_list: for each key, a 4-byte little-endian length followed by that many key bytes. This is the encoding the host (__frnt__delegate__list_secrets) writes into the delegate’s output buffer.

Kept in stdlib (rather than only in the host) so the format has exactly one authoritative definition that both sides — and the round-trip tests — share.