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/test/inject_generator_test.dart

@@ -480,9 +480,10 @@ void notAClass() {}
         );
       });
 
-      test('should generate empty mixin for class without @inject fields',
-          () async {
-        const input = '''
+      test(
+        'should generate empty mixin for class without @inject fields',
+        () async {
+          const input = '''
 import 'package:cherrypick_annotations/cherrypick_annotations.dart';
 
 part 'test_widget.inject.cherrypick.g.dart';
@@ -494,7 +495,7 @@ class TestWidget {
 }
 ''';
 
-        const expectedOutput = '''
+          const expectedOutput = '''
 // dart format width=80
 // GENERATED CODE - DO NOT MODIFY BY HAND
 
@@ -509,8 +510,9 @@ mixin _\$TestWidget {
 }
 ''';
 
-        await _testGeneration(input, expectedOutput);
-      });
+          await _testGeneration(input, expectedOutput);
+        },
+      );
     });
 
     group('Edge Cases', () {
@@ -593,12 +595,8 @@ mixin _\$TestWidget {
 Future<void> _testGeneration(String input, String expectedOutput) async {
   await testBuilder(
     injectBuilder(BuilderOptions.empty),
-    {
-      'a|lib/test_widget.dart': input,
-    },
-    outputs: {
-      'a|lib/test_widget.inject.cherrypick.g.dart': expectedOutput,
-    },
-    reader: await PackageAssetReader.currentIsolate(),
+    {'a|lib/test_widget.dart': input},
+    outputs: {'a|lib/test_widget.inject.cherrypick.g.dart': expectedOutput},
+    readerWriter: TestReaderWriter(),
   );
 }