refactor(generator): migrate cherrypick_generator to analyzer element2 API

- Fully migrated core cherrypick_generator and submodules to new analyzer element2 system:
  - Updated all GeneratorForAnnotation overrides to use Element2, ClassElement2, MethodElement2, FieldElement2 and new annotation/metadata access patterns.
  - Migrated signature and bodies for helpers, parsers, annotation validators, meta utils, and type parsers.
  - Fixed tests to use readerWriter instead of deprecated reader argument.
  - Refactored usage of now-absent 'metadata', 'parameters', 'fields', 'methods', 'source', and similar members to use correct *.firstFragment.* or API alternatives.
  - Cleaned up old imports and unused code.

test(generator): update generator integration tests

- Updated test calls to use correct TestReaderWriter type and bring test infra in line with current build_runner/testing API.

build: update dependencies and pubspec to support latest analyzer/build ecosystem

- Raised Dart SDK and package constraints as required for generated code and codegen plugins.
- Updated pubspecs in root/examples as needed by build warnings.

docs: add plots and assets (new files)

BREAKING CHANGE:
- Requires Dart 3.8+ and analyzer that supports element2.
- All downstream codegen/tests depending on Element API must migrate to Element2 signatures and data model.

cherrypick_generator/lib/src/generated_class.dart

@@ -12,6 +12,7 @@
 //
 
 import 'package:analyzer/dart/element/element.dart';
+import 'package:analyzer/dart/element/element2.dart';
 import 'bind_spec.dart';
 
 /// ---------------------------------------------------------------------------
@@ -75,14 +76,11 @@ class GeneratedClass {
   /// final gen = GeneratedClass.fromClassElement(classElement);
   /// print(gen.generatedClassName); // e.g. $AppModule
   /// ```
-  static GeneratedClass fromClassElement(ClassElement element) {
-    final className = element.displayName;
-    // Generated class name with '$' prefix (standard for generated Dart code).
+  static GeneratedClass fromClassElement(ClassElement2 element) {
+    final className = element.firstFragment.name2 ?? '';
     final generatedClassName = r'$' + className;
-    // Get source file name
-    final sourceFile = element.source.shortName;
-    // Collect bindings for all non-abstract methods.
-    final binds = element.methods
+    final sourceFile = element.firstFragment.libraryFragment.source.shortName;
+    final binds = element.methods2
         .where((m) => !m.isAbstract)
         .map(BindSpec.fromMethod)
         .toList();