Decouple FileStore from *os.File for object-store backends

The FileStore interface returned concrete *os.File from Open/Create/
OpenFile, which no non-filesystem backend (e.g. S3/R2) can produce, and
many file-library hot paths bypassed the interface entirely with direct
os.* / filepath.Walk calls.

Widen the interface to return io.ReadCloser / io.WriteCloser and add
ReadDir, ReadLink, and Walk so directory traversal no longer escapes the
abstraction. Route every file-library call site (fork writers, upload/
download handlers, GetFileNameList, CalcTotalSize/CalcItemCount, the set-
file-info folder rename) through the injected FileStore, and add a
WithFileStore option. OSFileStore keeps byte-identical behavior.

DownloadHandler now nil-guards the optional resource-fork reader instead
of relying on *os.File's nil-receiver tolerance, so a backend returning an
untyped-nil reader does not panic.

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