sliver_tree/_node_id_registry library

Internal: bidirectional key-to-nid mapping with free-list recycling.

Hands out stable integer handles ("nids") for arbitrary user keys so hot-path per-node state can live in dense typed-data arrays indexed by nid rather than hash maps keyed by the key type. Not exported from the package barrel: NodeStore owns the single instance and injects it into every component that maintains per-nid arrays.