Lets walkLogicalTree dispatch on multiple library shapes (video, movie,
audiobook, podcast) without proliferating boolean flags. Behavior for
existing video and movie libraries is unchanged; audiobook and podcast
modes will be consumed by the upcoming audiobook.go and podcast.go
parsers in later tasks of this sub-plan.
walkModeFor() derives the mode from a media_folders.type string;
unknown types default to walkModeVideo to preserve prior behavior for
any caller still passing a raw type.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
