Function decode_single_block 

pub fn decode_single_block(
    data: &[u8],
    block_idx: usize,
) -> Result<Vec<i64>, CodecError>

Decode a single block by index without decoding the entire stream.

Iterates block headers to reach block_idx, then decodes only that block. For sequential block-at-a-time processing, prefer BlockIterator which tracks byte offsets without re-scanning.