Incremental sync: --incremental flag to skip unchanged files

- New STATUS_CHECK protocol status (value 6)
- Client sends path + size + mtime; server replies OK (skip) or NEXT (send)
- config.h/c: use_incremental field sent/received over wire
- file.c/h: file_send_single_calls_no_path helper for incremental path
- client_cli.c: --incremental flag
- client_send.c: per-file check before send in both single and chunk paths
- server.c: STATUS_CHECK handling in receive_files()
- multiprocessing.c: STATUS_CHECK handling in receive_thread()
- test.py: incremental sync test case

src/shared/config.h

@@ -32,6 +32,7 @@ typedef struct Config {
   int include_count;
   unsigned long long max_size;
   unsigned long long min_size;
+  bool use_incremental;
 } Config;
 
 #define PROTOCOL_VERSION "1.0.0"