Add executable doctests documenting the consumer-facing contract of
irtt-proto: the directional request encode/decode/verify split, Params
construction and round-trip, omitted-vs-explicit-zero presence, open and
echo reply round trips, packet length calculation, and HMAC verification.
No conventional tests are migrated: every existing irtt-proto test
exercises wire vectors, flag matrices, malformed input, layout rules,
HMAC error behavior, or encode/decode round trips, which the crate's
AGENTS.md treats as durable compatibility coverage rather than
documentation cleanup targets.
Nils Andreas Svee
328ff17901