- FileClient composes FileStore/Searcher backends; Read()/Write()/Search()
select REST/DAV/PG/ES with transient read fallback. Client.Files() returns
it and *FileClient implements FileStore, so existing callers keep working.
- Entry gains backend-native Updated + folder FilesCount/FoldersCount so
dav ls output round-trips.
- cmd/oo dav/projects files and cmd/office/fetch download/preview/delete go
through FileStore; oo search goes through the facade.
- Mark transport methods that FileStore now abstracts as deprecated.
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