Change warn to error for invalid state node

Editor assessment

The subject states the change precisely: invalid state nodes now log an error instead of a warning. The machine scope is a proper subsystem noun, and the chore type is acceptable for a diagnostics-severity tweak, though fix could equally apply. The from-to relationship is fully expressed in the headline, so no body is needed and none is present. No footers appear. The lowercase imperative phrasing is clean. The message tells a reader exactly what changed, where, and in which direction without opening the diff.