fix: scanner mid-directory resume bug, -c -s protocol fix, 50 MB test data

- Fix scanner: when chunk fills up mid-directory, save DIR handle so
  remaining files in that directory are not skipped (pre-existing bug)
- Fix -c -s: add STATUS_CHUNK to protocol, send it before chunk data,
  handle it on the server side for both single and multithreaded paths
- Extract chunk_serialize/chunk_deserialize from chunk_compress/decompress
- Remove dead declarations (file_receive_from_buffer, etc.)
- Fix memory leak in receive_file_receive (free -> data_destroy)
- test.py generates ~50 MB of test data across bulk files
- All 8 integration tests + 7 unit tests pass

src/client/scanner.h

@@ -3,8 +3,12 @@
 
 #include "chunk.h"
 #include "queue.h"
+#include <dirent.h>
+
 typedef struct {
   Queue *directories;
+  DIR *current_dir;
+  char *current_path;
 } DirectoryScanner;
 
 DirectoryScanner *directory_scanner_create(char *root_directory);