Caller-supplied; uniqueness is enforced by the engine
(DuplicateNodeError). The identifier namespace belongs to
the user's information model — names like cell:wb1:Sheet1:A1 or
asset:property-1:HVAC-3 — and must not be mixed with editor-
controller identifiers like controller:gridSelection:wb1. That
separation is non-negotiable: keeping selected ranges and asset
locations in different identifier namespaces is what makes the
engine teachable.
Stable identifier for a node within a single graph instance.