Function write_header 

pub fn write_header(
    data: &mut [u8],
    disc: u8,
    version: u8,
    layout_id: &[u8; 8],
) -> ProgramResult

Write a complete Hopper header to the beginning of data.

Writes disc, version, flags (zeroed), layout_id, and the audit-added schema_epoch = 1 (bytes 12..16). Returns Err if data is shorter than 16 bytes.