CNTabBarRouteObserver class

NavigatorObserver that lets CNTabBar auto-hide while a modal/sheet is presented over its route (Issue #31).

Why it exists: on iOS, CNTabBar is rendered as a native UITabBar inside a Flutter UiKitView. When a Flutter-rendered modal sheet is presented over the same route (e.g. via showCupertinoSheet, showCupertinoModalPopup, or showModalBottomSheet), Flutter's hybrid composition can leave the tab bar's UIView at a higher z-index than the modal's Flutter content — making Material TextFields inside the modal invisible and letting the tab bar bleed through during sheet drags. (Cupertino has the same trade-off: UITabBarController would solve it natively, but that requires owning the whole nav stack.)

What it does: tracks the depth of modal/popup/sheet/dialog routes on every navigator it's attached to, exposed via modalDepth. When depth > 0, CNTabBar (with the default autoHideOnModal: true) swaps its platform view for an empty SizedBox of the same height, mirroring what iOS does natively when a UIViewController presents a full-screen modal over a UITabBarController.

Setup (one line per app):

MaterialApp(
  navigatorObservers: [CNTabBarRouteObserver()],
  // ...
)

or for CupertinoApp:

CupertinoApp(
  navigatorObservers: [CNTabBarRouteObserver()],
  // ...
)

Without this observer registered, CNTabBar still renders correctly — it just won't auto-hide when modals are pushed over it, and you may hit the Issue #31 z-order glitch with Flutter-rendered modal content.