- 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