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

src/shared/file.h

@@ -32,5 +32,7 @@ size_t file_content_to_buffer(File *file);
 FileMetadata *file_metadata_create(struct stat *stats);
 void file_metadata_destroy(void *metadata);
 bool to_disk(const char *path, const void *data, unsigned long long data_size);
+bool file_save_to_disk(const char *root_directory, File *file);
+File *receive_incremental_check(int fd, Config *config, bool *skipped);
 
 #endif