navi: honor Screen.canPop across back/swipe pop paths

canPop was documented as gating the back button / swipe but was ignored by
ScreenBackHandler, XRouterBack.didPopRoute, and XRouter.canPop, so a stack
entry with canPop=false was still poppable by gesture or system back. Make it
authoritative in all three so a non-poppable pushed entry (e.g. an activation
screen) can't be backed out from under the user.

lib/src/navi/router.dart

@@ -77,7 +77,8 @@ class XRouter extends BackButtonDispatcher
   late final _overlayEntry = OverlayEntry(builder: _buildContent);
 
   bool get canPop {
-    if (stack.isNotEmpty) return true;
+    final top = stack.lastOrNull;
+    if (top != null) return top.canPop;
     if (home is ScreenShell) {
       for (final screen in (home as ScreenShell).pages) {
         if (!screen.popped) return true;
@@ -225,7 +226,7 @@ class XRouterBack extends RootBackButtonDispatcher {
     }
     if (router.home.handleBack()) return true;
     final page = router.stack.lastOrNull;
-    if (page != null) {
+    if (page != null && page.canPop) {
       page.pop();
       return true;
     }

lib/src/navi/screen_stack.dart

@@ -75,7 +75,7 @@ class ScreenStackState extends State<ScreenStack> with TickerProviderStateMixin
   }
 
   void _updateCanPop() {
-    final canPop = _entries.any((e) => !e.removing);
+    final canPop = _entries.any((e) => !e.removing && e.page.canPop);
     if (canPop == _canPop) return;
     _canPop = canPop;
     WidgetsBinding.instance.addPostFrameCallback((_) {
@@ -258,7 +258,7 @@ class ScreenStackState extends State<ScreenStack> with TickerProviderStateMixin
               ScreenBackHandler(
                 key: entry.key,
                 entry: entry,
-                enabled: entry == top && _canPop,
+                enabled: entry == top && _canPop && entry.page.canPop,
                 onPop: () => entry.page.pop(),
               ),
           ],