Function encode 

pub fn encode(
    list: &PostingList,
    postings_per_block: u16,
    block_maxima: &[u8],
) -> Result<EncodedPostings, PostingsError>

Encodes one posting list in the original version 1 layout.

block_maxima supplies the per-block reserved u8 slot; pass an empty slice to store zeros. The impact-pair bytes stay zero, which is what keeps the frozen v1 golden byte-identical.

New writers use encode_v2. This entry point exists so the v1 golden stays exercised and so a v1 stream can still be produced deliberately.

Errors

Returns PostingsError::ZeroBlockSize when the geometry is degenerate.