Function encode_nt_payload_delta 

pub fn encode_nt_payload_delta(
    prev: &NtPayload,
    next: &NtPayload,
    filtered_desc: &StructureDesc,
    is_be: bool,
) -> Option<(Vec<u8>, Vec<u8>)>

Encode a sparse monitor-data delta between prev and next projected onto filtered_desc. Returns None if nothing changed in the filtered view (caller should suppress the update). Otherwise returns (bitset, values) with only the changed leaves marked and encoded.