put_u8/16/32/64 and get_u8/16/32/64 (plus signed variants) for packing
and unpacking values at explicit byte offsets in a buffer. Each value is
encoded byte-by-byte with explicit shift operations — no struct casting,
no alignment assumptions, correct on any host endianness.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
