names: parse the grouping folder, not the first path component

[cursor]

Bug and impact

Junk-tag Audible searches parsed the first folder under source_path as the release name. Two common layouts then searched for the wrong title and cached a miss (the book marker never expires):

1. Author/Title — Freida McFadden/The Guest/file.m4b parsed Freida McFadden as the title.
2. Category/Release — completed/The Guest/file.m4b (typical qBittorrent category folder when source_path is the downloads root) parsed completed as the title.

cd1/ / MP3/ / 64k/ wrappers under a real release folder were already fine (Some Release/cd1/ still parsed Some Release).

Root cause

releaseNameForBook took rel.split(os.sep)[0]. That matches Release/file and Release/cd1/file, but not Author/Title or a category folder. Grouping already keys on the title/release folder (groupingName); parsing did not.

Fix

releaseNameForBook now returns groupingName(...) — the same folder used as the book key — and still falls back to the grouping key for a loose file or a path outside the source.

Validation

• Unit tests for Author/Title, completed/Release, and Author/Title/cd1.
• Matching test: junk AudioTrack 01 tags under those layouts search title:The Guest, not the author or completed.
• Existing Release/cd1 and loose-file cases unchanged.
• python3 -m unittest discover -s tests -p 'test_*.py': 222 OK.
• Correctness and security review of the diff: no findings.