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

test_data/source/nested/another.txt

@@ -0,0 +1,50 @@
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file
+another nested file

test_data/source/nested/subdir/deep.txt

@@ -0,0 +1 @@
+deeply nested file