Immutable database values: time views, covering-index access, naming, per-attribute statistics, and bootstrap metadata.
A [Db] is a value: cheap to clone, never mutated in place. Time views
([Db::as_of], [Db::since], [Db::history]) wrap the same recorded
datoms with a different fold policy — no copying of facts. The four
covering indexes for a view are materialized lazily on first read and
shared by every clone of that value.