Persistent namespaced key/value store backed by a JSON file at
~/.local/share/cliamp/plugins/<name>/store.json. Values round-trip through the
existing Lua<->JSON conversion, so tables/numbers/strings/bools survive a
restart. Scoped per plugin (no cross-plugin reads); no permission required.
Adds appdir.DataDir() for non-config state.
Bjarne Øverli