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

test.py

@@ -383,6 +383,34 @@ def run_profile(profile_name, source_dir, dest_dir):
         except Exception as e:
             results.append({"name": "Progress (--progress)", "suite": profile_name, "status": "Error", "time": "N/A", "error": str(e)})
 
+        # Incremental sync (--incremental) — first sync, then second sync should skip all
+        print(f"\n  --- Incremental (--incremental) ---")
+        try:
+            flags = BASE_CLIENT_FLAGS + ["-M"]
+            srv = subprocess.Popen(SERVER_CMD, stdout=subprocess.DEVNULL, stderr=None)
+            time.sleep(0.5)
+            first_cmd = client_prefix + BASE_CLIENT_CMD + ["--source-dir", source_dir, "--dest-dir", dest_dir] + flags
+            r1 = subprocess.run(first_cmd, text=True, capture_output=True)
+            wait_proc(srv)
+            if r1.returncode != 0:
+                raise RuntimeError(f"First sync failed: {r1.stderr[:100]}")
+            # Second sync with --incremental should take less time
+            srv2 = subprocess.Popen(SERVER_CMD, stdout=subprocess.DEVNULL, stderr=None)
+            time.sleep(0.5)
+            second_cmd = client_prefix + BASE_CLIENT_CMD + ["--source-dir", source_dir, "--dest-dir", dest_dir] + flags + ["--incremental"]
+            start = time.monotonic()
+            r2 = subprocess.run(second_cmd, text=True, capture_output=True, timeout=30)
+            duration = time.monotonic() - start
+            wait_proc(srv2)
+            r = {"name": "Incremental (--incremental)", "suite": profile_name,
+                 "status": "Success" if r2.returncode == 0 else "Failed",
+                 "time": f"{duration:.4f}s" if r2.returncode == 0 else "N/A",
+                 "error": "" if r2.returncode == 0 else f"Exit {r2.returncode}: {(r2.stderr or r2.stdout)[:60]}"}
+            results.append(r)
+        except Exception as e:
+            results.append({"name": "Incremental (--incremental)", "suite": profile_name,
+                            "status": "Error", "time": "N/A", "error": str(e)})
+
         # Chunk size (--chunk-size 5242880)
         feature_flags = BASE_CLIENT_FLAGS + ["--chunk-size", "5242880"]
         cmd = client_prefix + BASE_CLIENT_CMD + ["--source-dir", source_dir, "--dest-dir", dest_dir] + feature_flags