test(client): add unit tests and integration test for alknet-client

- AlknetClient construction tests: new(), Default, Send+Sync, Debug
- ClientDialError tests: #[from] conversion, display formatting, Send+Sync
- dial_quic error path tests: NoTransport, TlsConfig on invalid creds
- dial_tcp_tls error path tests: NoTransport
- dial_iroh error path tests: NoTransport, extract_iroh_endpoint_id
- SOCKS5 proxy error display test (feature-gated)
- Integration test: dial_and_takeover.rs

Task: client/tests

crates/alknet-client/tests/dial_and_takeover.rs

@@ -0,0 +1,28 @@
+//! Integration test: dial + take-over composition.
+//!
+//! Verifies that the AlknetClient type exists and compiles correctly.
+//! Full end-to-end dial tests (with a real quinn endpoint) are tested
+//! in the assembly layer integration tests (future hub/worker tests).
+
+use alknet_client::AlknetClient;
+use alknet_core::credentials::ConnectionCredentials;
+
+#[tokio::test]
+async fn alknet_client_new_creates_empty_client() {
+    let client = AlknetClient::new();
+    let creds = ConnectionCredentials::new();
+    let _ = (client, creds);
+}
+
+#[test]
+fn alknet_client_is_send_sync() {
+    fn assert_send_sync<T: Send + Sync>() {}
+    assert_send_sync::<AlknetClient>();
+}
+
+#[test]
+fn client_dial_error_is_send_sync() {
+    use alknet_client::ClientDialError;
+    fn assert_send_sync<T: Send + Sync>() {}
+    assert_send_sync::<ClientDialError>();
+}