Subset of CDK's CdkStep carrying the _completedOverrideWritableSignal. MatStep.completed's getter reads this
signal, so cngx's per-step state computed transitively tracks
it through the getter and re-fires whenever Material flips
completion. Documented limitation: hasError writes that are
not paired with a completed change do not retrigger the
computed — sharper note lives on createMatStepHandle's JSDoc.