Function agent_doc_replica_diff 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn agent_doc_replica_diff(
    replica_id: u64,
    their_sv: *const u8,
    their_sv_len: usize,
    out_len: *mut usize,
) -> *mut u8

The incremental update replica replica_id should send a peer whose state vector is their_sv — only the ops that peer is missing (a delta, not a snapshot). Writes length to out_len. Returns null on not-open / bad-sv / poison. Free with agent_doc_free_state.

Safety

their_sv must point to their_sv_len readable bytes; out_len writable or null.