fix: keep redline outputs under output-dir on Windows rooted paths

Path.is_absolute() is False on Windows for a rooted-but-driveless path like
'/x/y', yet joining it onto output_dir still replaces the whole prefix, so
the escape guard never fired (caught by the new unit test on the
windows-latest CI matrix). Guard on drive/root/'..' components as well, and
cover '..' traversal in the test.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0142X9VTMpdo639kHKQa4PE1

Author: claude

action/redline_changed.py

@@ -276,8 +276,10 @@ def revision_count_from_stdout(stdout: Optional[str]) -> Optional[int]:
 def redline_output_paths(output_dir: str, source_path: str) -> Tuple[Path, Path]:
     """Mirror the source's relative directory tree under output_dir."""
     rel = Path(source_path)
-    if rel.is_absolute():
-        # Joining an absolute path onto output_dir would escape it entirely.
+    # An absolute path joined onto output_dir would replace it entirely, and a
+    # '..' component would escape it. Note is_absolute() alone is not enough on
+    # Windows, where a rooted-but-driveless path ('/x/y') still hijacks a join.
+    if rel.is_absolute() or rel.drive or rel.root or '..' in rel.parts:
         try:
             rel = rel.relative_to(Path.cwd())
         except ValueError:

tests/test_action_script.py

@@ -118,6 +118,9 @@ def test_redline_output_paths_absolute_source_stays_under_output_dir(tmp_path, m
     outside = '/somewhere/else/entirely/b.docx'
     docx, _ = ra.redline_output_paths('out', outside)
     assert docx.as_posix() == 'out/b.redline.docx'
+    traversal = '../outside/c.docx'
+    docx, _ = ra.redline_output_paths('out', traversal)
+    assert docx.as_posix() == 'out/c.redline.docx'
 
 
 def test_build_summary_lists_each_change():