One entry of a live navigation stack — a screen and its id. T is the
screen representation: the raw spec enum internally, or the public
Screen<Object?> wrapper for the consumer-facing stack.
A live navigation stack, root-first: the full record (screens + ids) plus
derived views. The one stack type — the engine fills it with raw screens
for onImpossiblePop, the generated Screen.stack fills it with wrappers.
Also the per-scope building block restoration serializes.
Marks the library-private spec enum the nav generator reads.
Rows: identifier = screen name, first positional = const widget,
optional second positional = id Type (named types only).