`split --out-dir DIR` wrote each part with `std::fs::write` and never
created DIR, so a folder that did not exist yet failed on the first part
with the part's path and a raw OS error ("The system cannot find the path
specified. (os error 3)") — easy to read as the split itself failing.
The folder is now created with `create_dir_all` once the parts have been
computed, before any is written; a folder that can't be created is
reported as `split: --out-dir DIR: <error>`, naming the folder rather
than the first part.
Tests (apps/pdfcraft-cli/tests/split.rs): a nested folder that does not
exist is created and holds every part listed on stdout; an `--out-dir`
that is an existing file fails with a message naming the option and
leaves the file as it was.
Co-authored-by: wstock <1394687+wstock@users.noreply.github.com>
Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>
Co-authored-by: Brandon Thomas <echelon@gmail.com>
wstock