refactor: deduplicate server receive logic, extract shared helpers

- Extract file_save_to_disk() helper (replaces 4x duplicated disk-save boilerplate)
- Extract receive_incremental_check() shared helper (deduplicates STATUS_CHECK
  handling between server.c single-threaded and multiprocessing.c multi-threaded paths)
- Break receive_files() into receive_chunk() and receive_manifest() sub-handlers
- Add incremental sync test case to test.py
- Rebase onto main

test.py

@@ -383,17 +383,6 @@ 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)})
 
-        # Bandwidth limit (--bwlimit 10240 = 10 MB/s)
-        feature_flags = BASE_CLIENT_FLAGS + ["--bwlimit", "10240"]
-        cmd = client_prefix + BASE_CLIENT_CMD + ["--source-dir", source_dir, "--dest-dir", dest_dir] + feature_flags
-        print(f"\n  --- Bandwidth limit (--bwlimit 10240 KB/s) ---\n    Running: {' '.join(cmd)}")
-        try:
-            r = run_single_test(cmd, "Bandwidth limit (--bwlimit 10240)", source_dir, dest_dir)
-            r["suite"] = profile_name
-            results.append(r)
-        except Exception as e:
-            results.append({"name": "Bandwidth limit (--bwlimit 10240)", "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: