- Add file_send_sendfile() using sendfile() syscall to send file
content directly from fd to socket, bypassing userspace memory
- Add use_sendfile field to Config struct (default false)
- Add -f / --sendfile flag parsing in client.c
- sendfile path in send_chunk() used when -f is set without -c or -s
- send_files_multithreaded() falls back to single-threaded when
sendfile is enabled (loader pipeline becomes unnecessary)
- Add Sendfile (-f) test case to test.py