A simple hashmap-backed [DataStore].
The lean, dependency-free reference implementation of
[arora_types::data::DataStore]: a HashMap of path-keyed cells, std
channels for change subscriptions. Cheaply cloneable — clones share the same
storage, so the same blackboard can be handed to the HAL, the bridge, the
behavior tree, and the engine at once. Richer backends (e.g. arora-ecbs) can
implement the same trait.