Respect the file.data.unlisted convention in both the folder discovery
loop (virtual folder index generation in pageType.generate) and the
folder listing itself (pagesFromAllFiles and pagesFromTrie in
FolderContent).
Unlisted pages no longer contribute folders or subfolders, and no longer
appear as entries inside any folder page listing. The foldersWithIndex
detection loop also skips unlisted pages so a user-authored unlisted
index file does not bypass the filter.
Exports pagesFromAllFiles so its filter logic can be tested directly.
Adds 5 unit tests covering unlisted exclusion, explicit unlisted: false
passthrough, subfolder grouping, and folder-index self-exclusion.
