feat: add -f/--sendfile for zero-copy file transfer

- 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

test.py

@@ -48,6 +48,7 @@ TEST_CASES = [
         "name": "Multithreading + Compression + Chunk Serialization (-m -c -s)",
         "flags": ["-m", "-c", "-s"],
     },
+    {"name": "Sendfile (-f)", "flags": ["-f"]},
 ]