Added an e2e test harness with a fake R701 device

tests/mock/mod.rs emulates the device's TCP protocol (ping, get_name,
get_total_record_count, get_record_bytes), grounded in the reverse
engineering notes at
https://nicolabelluti.me/posts/2024/05/studying-a-communication-protocol/,
so tests/e2e.rs can exercise the real R701 client against it instead of
needing a physical reader. Starts with one test (connect + ping);
more scenarios (get_name, record streaming, error cases) to follow.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/e2e.rs

@@ -0,0 +1,14 @@
+//! End-to-end tests exercising R701's public API against the emulated
+//! device in `mock`.
+
+#![allow(clippy::unwrap_used)]
+
+mod mock;
+
+use r701::R701;
+
+#[tokio::test]
+async fn connects_and_pings_successfully() {
+    let addr = mock::spawn().await;
+    assert!(R701::connect(addr).await.is_ok());
+}