Function write_initial_auxv 

pub fn write_initial_auxv(
    prng: &mut Pcg64Mcg,
    memory: impl MemoryAccess,
    pointer: AddrMut<'_, u8>,
) -> Result<[u8; 16], Errno>

Draw and write the initial auxv bytes before the thread’s post-exec callback, emitting no record, and return them for the handoff (encode_initial_state). Post-exec then emits the record, so a run’s records come in the same order whichever backend wrote the bytes, and the root thread’s seeding records precede it on every backend.