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/module_generator.dart

@@ -11,7 +11,7 @@
 // limitations under the License.
 //
 
-import 'package:analyzer/dart/element/element.dart';
+import 'package:analyzer/dart/element/element2.dart';
 import 'package:build/build.dart';
 import 'package:source_gen/source_gen.dart';
 import 'package:cherrypick_annotations/cherrypick_annotations.dart' as ann;
@@ -79,12 +79,12 @@ class ModuleGenerator extends GeneratorForAnnotation<ann.module> {
   ///
   /// See file-level docs for usage and generated output example.
   @override
-  String generateForAnnotatedElement(
-    Element element,
+  dynamic generateForAnnotatedElement(
+    Element2 element,
     ConstantReader annotation,
     BuildStep buildStep,
   ) {
-    if (element is! ClassElement) {
+    if (element is! ClassElement2) {
       throw InvalidGenerationSourceError(
         '@module() can only be applied to classes.',
         element: element,