fix(tests): break the irtt-client/irtt-server dev-dependency cycle

irtt-client and irtt-server mutually dev-depended on each other for
cross-crate integration tests. That cycle blocks `cargo publish` for
either crate whenever the other doesn't already exist on the registry
at the required version, which is exactly the situation for a crate's
first-ever release (irtt-server has never been published before).

Introduce a new `irtt-integration-tests` crate (publish = false, no
library target) to own tests whose actual subject is the interaction
between two or more irtt-* production crates:

  crate-local behavior          -> crate's own tests/
  cross-crate/system behavior   -> irtt-integration-tests

Move every test that imports both irtt_client and irtt_server there:
the client-side support harness (hmac, large_packets, protocol_options,
fill_negotiation) and the server-side real-socket tests that drive a
real AsyncClient against a real Server (async_client, negotiation_policy,
fill_wire_bytes, traffic_class, wildcard_source). server_set.rs stays in
irtt-server/tests since it only uses irtt_proto as test machinery to
drive irtt-server directly, which is a normal one-directional test
dependency, not a peer relationship.

irtt-server's dev-dependency on irtt-client is dropped entirely now
that nothing under crates/irtt-server/tests needs it, which fully
breaks the cycle from that side.

irtt-client keeps a one-directional dev-dependency on irtt-server: a
handful of white-box unit tests in src/async_client/tests.rs assert on
private AsyncClient internals (socket, remote, test_hooks) while also
needing a real in-tree server for blocking/async equivalence checks, so
they can't move to an external crate without losing that access. Their
InTreeServer helper is duplicated locally rather than shared, matching
the existing precedent of irtt-cli's own tests/support/in_tree_server.rs.

crates/irtt-integration-tests/tests/large_packets.rs

@@ -0,0 +1,103 @@
+mod support;
+
+use irtt_client::{Client, ClientConfig, ClientEvent, NegotiationPolicy};
+use irtt_proto::{echo_header_len, Params, TimestampFields};
+
+use support::{config_for_params, default_params, run_one_probe, BackendPeer, ServerObservation};
+
+use crate::support::test_echo_packet_len;
+
+fn params_with_length(length: i64) -> Params {
+    Params {
+        length,
+        ..default_params()
+    }
+}
+
+fn assert_echo_size(run: &support::OneProbeRun, expected: usize) {
+    match run.observations[1] {
+        ServerObservation::Echo { len, .. } => assert_eq!(len, expected),
+        ref other => panic!("expected Echo observation, got {other:?}"),
+    }
+}
+
+fn assert_reply(event: &ClientEvent) {
+    assert!(matches!(event, ClientEvent::EchoReply { .. }));
+}
+
+fn assert_sent_bytes(event: &ClientEvent, expected: usize) {
+    match event {
+        ClientEvent::EchoSent { bytes, .. } => assert_eq!(*bytes, expected),
+        other => panic!("expected EchoSent, got {other:?}"),
+    }
+}
+
+fn assert_reply_bytes(event: &ClientEvent, expected: usize) {
+    match event {
+        ClientEvent::EchoReply { bytes, .. } => assert_eq!(*bytes, expected),
+        other => panic!("expected EchoReply, got {other:?}"),
+    }
+}
+
+// ─── Echo request size: FakeServer observes exact packet size ───
+
+#[test]
+fn echo_request_size_matches_echo_packet_len_for_zero_length() {
+    let params = params_with_length(0);
+    let run = run_one_probe(params.clone(), TimestampFields::default());
+    let expected = test_echo_packet_len(false, &params);
+    assert_echo_size(&run, expected);
+    assert_sent_bytes(&run.sent, expected);
+}
+
+#[test]
+fn echo_request_size_matches_echo_packet_len_above_header() {
+    for length in [16, 60, 128, 1472, 4096] {
+        let params = params_with_length(length);
+        let run = run_one_probe(params.clone(), TimestampFields::default());
+        let expected = test_echo_packet_len(false, &params);
+        assert_eq!(
+            expected,
+            echo_header_len(false, &params).max(length as usize)
+        );
+        assert_echo_size(&run, expected);
+        assert_sent_bytes(&run.sent, expected);
+    }
+}
+
+// ─── Echo reply handling ───
+
+#[test]
+fn echo_reply_bytes_consistent_with_echo_packet_len_for_various_lengths() {
+    for length in [0, 1472, 4096] {
+        let params = params_with_length(length);
+        let run = run_one_probe(params.clone(), TimestampFields::default());
+        let expected = test_echo_packet_len(false, &params);
+        assert_reply(&run.reply);
+        assert_reply_bytes(&run.reply, expected);
+    }
+}
+
+// ─── Backend-neutral smoke ───
+
+#[test]
+fn backend_large_packet_smoke() {
+    let params = params_with_length(1472);
+    let peer = BackendPeer::start_open_echo(None);
+    let mut client = Client::connect(ClientConfig {
+        negotiation_policy: NegotiationPolicy::Loose,
+        length: 1472,
+        ..config_for_params(peer.addr(), &params)
+    })
+    .unwrap();
+
+    let outcome = client.open().unwrap();
+    assert!(matches!(outcome, irtt_client::OpenOutcome::Started { .. }));
+
+    client.send_probe().unwrap();
+    let events = client.recv_once().unwrap();
+    assert_eq!(events.len(), 1);
+    assert!(matches!(events[0], ClientEvent::EchoReply { .. }));
+
+    client.close().unwrap();
+}