GetStores replaces GetDCFStores: it takes a context, has no count
argument and returns a plain slice. GetStore returns the one mounted
card, or ErrNoStore or ErrMultipleStores.
Each Object keeps the fs.FS its card was read through, and Hash reads
through it, so tests can use fstest.MapFS. A cancelled context stops
the read before the next directory. Library errors wrap with %w.
Model: opus-5-5
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