examples/usercmodule/iperf3_lwip: Refactor to unified transfer() API.

Implemented unified transfer() function with mode parameter to work around
3-function module globals table limit on OpenMV N6:
- transfer('tcp_client', host, port, duration): TCP client mode
- transfer('tcp_server', port, duration): TCP server mode
- transfer('udp_client', host, port, duration, bandwidth): UDP client
- transfer('udp_server', port, duration): UDP server

Also supports integer mode codes (0-3) for simplified API.

Added UDP support with iperf3-compatible packet format:
- 12-byte header: sec, usec, seq in network byte order
- RFC 1889 jitter calculation
- Packet loss detection from sequence gaps
- Bandwidth limiting via inter-packet delay

Tested on OpenMV N6 (STM32N657 @ 800MHz):
- Module boots successfully with unified API
- TCP modes confirmed working (previously 474 Mbits/sec)
- UDP modes implemented, testing pending

Resolves 3-function module limit documented in CLAUDE.local.md.

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

Author: pi-anl