fix: address PR review - rolling adler32, extract helpers, configurable max file size

- Fix rolling adler32: add missing -1 in s2 update formula (was producing
  wrong checksums, causing zero block matches)
- Fix file_send_sendfile signature to match typedef (add unused
  compression_level param)
- Extract receive_delta_file() from receive_incremental_check() to reduce
  nesting depth
- Extract send_file_incremental() helper in client_send.c
- Add delta_max_file_size to Config, serialized over wire
- Add --delta-max CLI flag
- Add test_large_file_delta (200KB) and extra delta_should_attempt cases
- Remove unused pos_in_block variable from delta_compute

src/shared/file.h

@@ -25,7 +25,7 @@ void file_destroy(void *item);
 bool file_load_data(File *file);
 File *file_receive(Config *config, int file_descriptor);
 bool file_send_single_calls(File *file, int file_descriptor, bool use_metadata, int compression_level, bool send_path);
-bool file_send_sendfile(File *file, int file_descriptor, bool use_metadata, bool send_path);
+bool file_send_sendfile(File *file, int file_descriptor, bool use_metadata, int compression_level, bool send_path);
 size_t file_content_to_buffer(File *file);
 FileMetadata *file_metadata_create(struct stat *stats);
 void file_metadata_destroy(void *metadata);