Follow-up to #1237. \`rename_and_group\` only grouped when the source
root was a directory, so a multi-file audiobook delivered as a single
archive fell through to the flat path: a \`Book.zip\` of twelve chapters
landed loose in the destination root with its original chapter names —
the layout #1181 is about.
The \`is_dir()\` guard was there to keep \`Book.zip/\` from becoming the
folder name, but skipping the file case gives up the grouping instead of
naming it. A non-directory source can only produce several book files by
having been extracted (\`collect_staged_files\` returns a single-element
list for every other file shape), so the archive stem is the release
name and the suffix is packaging: group under \`Book/\`.
Also regenerates the env docs for the new option and gives it the same
\"do not use with ingest folders\" caveat Rename and Organize carries,
since both now create directories in the destination.
Tested: reverting only the source fix makes both new tests fail and the
\`rename\` control case pass, so grouping stays opt-in. Full non-e2e
suite green (2653 passed).
CaliBrain
7d56624ab6