Implement `cmd/test-client/main.cpp` UDP send/receive loop

[justintulloch]

• Parse flags:
– --serverAddr (e.g. localhost:20000)
– --clientID (e.g. client-1)
– --jitter (int, max ms)
– --lossPct (double, 0.0–1.0)
• Every 50 ms:

1. Build an InputEvent (with incrementing SequenceNum), serialize.

2. Sleep a random amount up to jitter ms.

3. With probability lossPct, drop (skip send). Otherwise, send UDP to serverAddr.

   • Concurrently, listen on UDP port :60000 for WorldState messages (C++ serialization), decode, and std::cout << the tick and positions.

   • Exit cleanly on SIGINT, printing summary stats (packets sent, packets lost, ticks received).