Enough of @agoric/store's MapStore and @endo/patterns' M for the
credential-rotation design: makeMapStore(label, { keyShape, valueShape },
kv) keeps entries in a DO's sync KV under store:<label>:, and readOnly()
hands out the queries alone. Facets are workerd RpcTargets in place of
endo's Far.
Unlike endo, error messages never include keys or values: the values
here are credentials, and messages cross RPC into logs.
Generated-by: Claude Opus 5.5
