Add `MetadataStore` (src/library/store.ts): one metadata.json holding the
user id, schema version, collections cursor, and decrypted collection and
file records. Loaded whole into RAM with Maps for id lookups; rewritten
whole through the download layer's fsync atomic writer (temp, fsync,
rename, dir fsync). Binary keys are base64-encoded on disk. A missing,
unparseable, or wrong-schema file loads as an empty store, because the
file is only a cache the refresh unit repopulates. Directory 0700, file
0600, matching session.json. No lock file; no sync() beyond the writer.
This unit only stores; issue 42's refresh unit populates it. (closes#41)
Model: opus-4-8
