phase3: nullfix.py batch script — bang/late/param-nullable

Automated null-safety fixes driven by dart analyze (no corruption):
- unchecked_use_of_nullable_value: insert '!' on receiver (property/method/[]/op)
- not_initialized field/var: mark 'late'
- missing_default_value_for_parameter: nullable param
Errors: 2374(peak) -> 907

lib/pages/plain_page.dart

@@ -23,7 +23,7 @@ class PlainPage extends StatefulWidget {
 }
 
 class PlainPageState extends State<PlainPage> {
-  Blog blog;
+  late Blog blog;
 
   @override
   Widget build(BuildContext context) {