- _long_path: UNC paths (\\server\share\...) now use the \\?\UNC\ form
  instead of plain \\?\, which produced invalid \\?\\\server\share and
  caused WinError 123 on extraction to SMB shares.
- _batch_by_size: flush current batch *before* appending an oversize
  entry, so small files + one huge file no longer bundle into one batch.
- Strip trailing `"` from Windows source/destination args to recover
  from cmd.exe's `\"` escape of quoted paths ending in backslash.