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

src/shared/config.h

@@ -11,6 +11,7 @@ typedef struct Config {
   bool use_multithreading;
   bool use_chunk_serialization;
   bool use_compression;
+  bool use_sendfile;
   bool use_single_send_per_file;
   int compression_level;
   int num_connections;