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/shared/socket.c

@@ -195,6 +195,8 @@ const char *status_to_string(Status status) {
     return "FINISHED";
   case STATUS_NEXT:
     return "NEXT";
+  case STATUS_CHUNK:
+    return "CHUNK";
   default:
     return "UNKNOWN";
   }