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>

Cargo.toml

@@ -23,3 +23,6 @@ unwrap_used = "deny"
 enum_glob_use = { level = "deny", priority = 1 }
 pedantic = { level = "deny", priority = -1 }
 nursery = { level = "deny", priority = -1 }
+
+[dev-dependencies]
+tokio = { version = "1.53.1", features = ["rt", "macros", "net"] }