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.

examples/client_app/pubspec.yaml

@@ -5,7 +5,7 @@ publish_to: 'none'
 version: 1.0.0+1
 
 environment:
-  sdk: ">=3.5.2 <4.0.0"
+  sdk: ">=3.8.0 <4.0.0"
 
 
 dependencies:
@@ -25,11 +25,11 @@ dev_dependencies:
   flutter_test:
     sdk: flutter
 
-  flutter_lints: ^5.0.0
+  flutter_lints: ^6.0.0
 
   cherrypick_generator:
     path: ../../cherrypick_generator
-  build_runner: ^2.4.15
+  build_runner: ^2.5.0
 
 # For information on the generic Dart part of this file, see the
 # following page: https://dart.dev/tools/pub/pubspec