Second of six sub-plans. 10 tasks: PersonKind constants for Author and
Narrator, audio-extension recognizer, library-type helpers, a
walkLogicalTree refactor (movieLibrary bool -> typed walkMode), chapter
extraction via ffprobe, single-file and multi-file audiobook parsers,
scanner write path producing media_items.type='audiobook', and a
filesystem podcast parser (RSS deferred to sub-plan 5).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
