In-memory property graph runtime.
The graph crate owns node/edge storage, label sets, property maps, directed
adjacency, built-in label/property indexes, typed mutation validation, and
the CORE persistence provider. SharedGraph serializes writes through a
transaction boundary while readers observe immutable snapshots. selene-db is
a single native engine: the higher selene-gql layer owns GQL
binding/planning and the one frozen native procedure registry.