docs(irtt-proto): add runnable doctests for public API

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.

crates/irtt-proto/src/hmac.rs

@@ -68,6 +68,23 @@ pub fn verify_hmac(key: &[u8], packet: &[u8], hmac_offset: usize) -> Result<()>
 /// [`ProtoError::InvalidHmacOffset`] when the field is truncated, and
 /// [`ProtoError::BadHmac`] when the MAC does not match. The packet is never
 /// modified.
+///
+/// # Example
+///
+/// ```
+/// use irtt_proto::{encode_request, verify_packet_hmac, Params, RequestToEncode};
+///
+/// // Synthetic example key — use a real shared secret in production.
+/// let key = b"example key";
+/// let params = Params::with_protocol_defaults();
+/// let packet = encode_request(
+///     RequestToEncode::Open { params: &params, no_test: false },
+///     Some(key),
+/// ).unwrap();
+///
+/// verify_packet_hmac(key, &packet).unwrap();
+/// assert!(verify_packet_hmac(b"wrong key", &packet).is_err());
+/// ```
 pub fn verify_packet_hmac(key: &[u8], packet: &[u8]) -> Result<()> {
     let envelope = envelope::decode_structural(packet)?;
     if !envelope.hmac_present {