Add @State, StateBox, Binding, DependencyTracker, and
SubscriptionToken for targeted reactive updates.
Label gains two reactive overloads: Label(Binding<String>)
for the explicit-binding path and Label(() -> String) for
tracked closures. Both update the Gtk.Label text in-place
without re-mounting.
Regression tests verify label updates survive when the
Swift wrapper is deallocated after the GObject is attached
to the widget tree (GTK does not retain wrappers).
Includes 28 new tests across ReactiveLabelTests and
StateTests suites.
