Fix CLI stdout count report + add regression test

Devin Review caught that `--out -` discarded the sample count, so
the stderr summary always said 'wrote 0 sample(s)' even when bytes
were streamed. Capture the return value like the file-output branch
does, and add a regression test that exercises the stdout path.

Co-Authored-By: adrian kuman firmansah <adriancuman@gmail.com>

src/ocr_sprint/tools/export_ground_truth.py

@@ -101,9 +101,8 @@ def main(argv: list[str] | None = None) -> int:
     )
 
     out: Path = args.out
-    count = 0
     if str(out) == "-":
-        _write_stream(sys.stdout.buffer, filters)
+        count = _write_stream(sys.stdout.buffer, filters)
     else:
         out.parent.mkdir(parents=True, exist_ok=True)
         with out.open("wb") as fh: