Model files: derived arbor structure over any Quarb source.
A [Model] (parsed from a --model file, see [parse_model])
declares derived containers, references, and edges over a
base arbor — the top rung of the reference-provenance ladder,
generalizing the SQLite views-and---refs construction to any
substrate (a CSV, a JSON export, a log stream).
[ModelAdapter] wraps a base adapter and presents the base's own
nodes unchanged, adding the derived containers as new root
children in a reserved node-id band (bit 63). Derived structure
materializes lazily: a node constructor runs its query over the
base (never over self, so no recursion) on first touch, and
the distinct string-keyed values are cached. Declared references
and edges build forward/reverse value indexes on first use.