Wiring bundle returned from createMatTabHandle. The
directive owns the writable errorAggregator slot so the
[cngxMatTabError] per-tab directive can pump aggregators into
it; label and disabled track Material's current state via a
toSignal(matTab._stateChanges) trigger.
Direct invalid flag the [cngxMatTabErrorFlag] directive writes -
string | boolean, mirroring CngxTab.error. Folds into the
handle's hasError / errorMessage alongside the aggregator.