Schema 3 introduces multi-graph IR: a single exported document may
carry nodes, commits, and events that originated on more than one
engine instance once the future cross-graph aggregator lands. The
graphId field on every node and commit is the foreign key the
checker uses to partition passes per graph. Source rule (name
from createCausl({ name }) precedence + UUID v4 fallback) lives
in createCausl's implementation.
Stable identifier for a single graph instance.