Raised by {@see \LaravelUi5\Core\Ui5\Dashboard\DefaultDashboardTransformer}
when a dashboard group declares a child namespace
({@see \LaravelUi5\Core\Ui5\Contracts\Ui5DashboardGroupInterface::getChildNamespaces()})
that the registry can't resolve — the child artifact was never
registered on its module (its ARTIFACTS).
This turns the old "register in two places" footgun into a checked
invariant: a group can only compose artifacts that actually exist in
the registry, and the failure surfaces loudly at composition instead
of silently 404-ing the integration-card manifest fetch.