ocaml-space-packet: enable MDX on lib/space_packet.mli
Run mdx on lib/space_packet.mli so the {[ ... ]} odoc block now
type-checks. The example referenced free `codec`, `buf`, `off`;
qualified the codec names to `Space_packet.codec` /
`Space_packet.bf_apid`, allocated a real `buf = Bytes.make 6 '\x00'`,
and added an `assert (apid = 0)` so the zero-copy field-getter
example documents what an empty buffer reads.