Folders containing N audio files (one per chapter/part) get one
parsedAudiobookFile per file; each file's chapter list is synthesized
as a single chapter with title = filename stem. Title/author/series/
year come from the first file's tags.
Also drops the duplicate pickFirstNonEmpty helper added in Task 6 in
favor of the existing firstNonEmpty already in probe.go.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
