Subset of Material's MatTab carrying the _closestTabGroup
back-reference (the owning MatTabGroup, resolved via DI at
construction). Material's own _allTabs filter compares against
it for exactly the reason cngx does: a descendants: true query
also surfaces tabs of a nested <mat-tab-group>, and only the
ownership comparison separates them. Typed unknown because the
upstream declaration is any - cngx only ever identity-compares.