StateChangeSource

Subset of Material's MatTab carrying the _stateChanges Subject. The leading underscore signals Material-internal-by-convention, even though the field is readonly on the public class surface. The Subject emits void whenever a MatTab input (textLabel, disabled, …) changes; cngx bridges it into the Signal graph via toSignal(matTab._stateChanges) because Material exposes no public reactive surface for input changes.