examples/usercmodule/iperf3_lwip: Add TCP send/receive test module (WIP - boot failure).

Implemented direct lwIP PCB approach for high-performance network testing:
- tcp_send_test() for client TX mode
- tcp_recv_test() for server RX mode
- Heap-allocated 16KB buffer (lazy init)
- Zero-copy transmission with TCP_WRITE_FLAG_MORE
- TCP_NODELAY enabled for maximum throughput

Based on ciperf.c patterns achieving 489 Mbits/sec.

ISSUE: Device fails to boot after flashing (no USB enumeration).
Same boot failure encountered with ciperf initially - requires
investigation of static initialization or module registration.

Expected performance: 400-600 Mbits/sec when boot issue resolved.

Signed-off-by: Andrew Leech <[email protected]>

Author: pi-anl