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/pubspec.yaml

@@ -15,20 +15,20 @@ topics:
   - inversion-of-control
 
 environment:
-  sdk: ">=3.6.0 <4.0.0"
+  sdk: ">=3.8.0 <4.0.0"
 
 # Add regular dependencies here.
 dependencies:
-  cherrypick_annotations: ^3.0.1
-  analyzer: ^7.7.1
+  cherrypick_annotations: ^3.0.0
+  analyzer: ">=7.5.9 <8.0.0"
   dart_style: ^3.0.0
-  build: ^2.4.1
-  source_gen: ^2.0.0
+  build: ^3.0.0
+  source_gen: ^3.1.0
   collection: ^1.18.0
 
 dev_dependencies:
-  lints: ^5.1.1
+  lints: ^6.0.0
   mockito: ^5.4.5
   test: ^1.25.8
-  build_test: ^2.1.7
-  build_runner: ^2.4.13
+  build_test: ^3.0.0
+  build_runner: ^2.5.0