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/delta.h

@@ -72,7 +72,7 @@ void *delta_apply(const void *old_data, uint64_t old_size, const Delta *delta,
                   uint32_t block_size);
 void delta_destroy(Delta *delta);
 
-bool delta_should_attempt(uint64_t old_size, uint64_t new_size);
+bool delta_should_attempt(uint64_t old_size, uint64_t new_size, uint64_t max_file_size);
 bool delta_is_worthwhile(const Delta *delta, uint64_t new_file_size);
 
 uint32_t delta_adler32(const void *data, uint32_t len);