The edge-swipe back gesture now works on pages with custom transitions:
on iOS the drag drives the page's own transitionsBuilder in reverse and
pops past halfway or on a fling, like CupertinoPageRoute. New swipeBack
parameter (default true; false for .none) to opt out. Gesture respects
popGestureEnabled (root, fullscreenDialog, PopScope), handles RTL, and
the pop syncs the NavStack.