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/config.h

@@ -36,6 +36,7 @@ typedef struct Config {
   bool use_incremental;
   bool use_delta;
   uint32_t delta_block_size;
+  unsigned long long delta_max_file_size;
   bool use_tls;
   char *tls_cert;
   char *tls_key;