docs: restore testing infrastructure action plan

- Re-added ACTION_PLAN.md that was lost during branch cleanup
- Contains comprehensive 5-phase testing improvement strategy
- Documents 13 child branches with specific tasks and success metrics

Author: Michael Stelly

ACTION_PLAN.md

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+# Testing Infrastructure Improvement Action Plan
+
+## Phase 1: Foundation Fixes (Branch: `testing-infrastructure-improvements`)
+
+### Child Branch 1: `fix/jest-configuration`
+- **Issue**: Jest config syntax error preventing proper test execution
+- **Actions**:
+  - Fix `testMatch` pattern in jest.config.js (remove extra `>`)
+  - Verify test discovery works correctly
+  - Update test scripts if needed
+
+### Child Branch 2: `feat/test-coverage-reporting`
+- **Issue**: No coverage collection or reporting
+- **Actions**:
+  - Configure Jest coverage collection
+  - Add coverage scripts to package.json
+  - Set up coverage thresholds (start with 50% minimum)
+  - Generate HTML coverage reports
+  - Add coverage to CI pipeline
+
+### Child Branch 3: `feat/e2e-tests-in-ci`
+- **Issue**: E2E tests not running in GitHub Actions
+- **Actions**:
+  - Update push.yml workflow to include e2e tests
+  - Configure Docker environment for CI
+  - Add visual diff reporting for failed tests
+  - Set up artifact storage for screenshots
+
+## Phase 2: Core Component Testing (Branch: `testing-components`)
+
+### Child Branch 4: `test/main-page-components`
+- **Issue**: 0% coverage on critical components (Home, VersionSelector)
+- **Actions**:
+  - Add comprehensive tests for Home.tsx
+  - Test VersionSelector component interactions
+  - Test UpgradeButton functionality
+  - Add integration tests for version selection flow
+
+### Child Branch 5: `test/diff-viewer-components`
+- **Issue**: All 7 Diff components untested
+- **Actions**:
+  - Test DiffViewer.tsx rendering and state management
+  - Test DiffSection.tsx file filtering and display
+  - Test DiffHeader.tsx anchor links and actions
+  - Test Diff.tsx component interactions
+  - Add tests for collapse/expand functionality
+
+### Child Branch 6: `test/ui-components`
+- **Issue**: Missing tests for 16 remaining components
+- **Actions**:
+  - Test all button components (Copy, Download, View, etc.)
+  - Test CompletedFilesCounter interactions
+  - Test alert and notification components
+  - Test VersionSelector dropdown behavior
+
+## Phase 3: Hook and Data Layer Testing (Branch: `testing-hooks-data`)
+
+### Child Branch 7: `test/custom-hooks`
+- **Issue**: 0% hook coverage - critical data fetching untested
+- **Actions**:
+  - Test useFetchDiff hook with mock responses
+  - Test useFetchReleaseVersions hook
+  - Test URL parsing hooks (get-language-from-url, get-package-name-from-url)
+  - Add error handling tests for all hooks
+
+### Child Branch 8: `test/utils-and-helpers`
+- **Issue**: 67% of utility functions untested
+- **Actions**:
+  - Complete utils.ts test coverage
+  - Test device-sizes.ts utility functions  
+  - Test update-url.ts URL manipulation
+  - Add edge case testing for all utilities
+
+## Phase 4: Advanced Testing Features (Branch: `testing-enhancements`)
+
+### Child Branch 9: `feat/accessibility-testing`
+- **Issue**: No accessibility testing infrastructure
+- **Actions**:
+  - Add jest-axe dependency
+  - Create accessibility test utilities
+  - Add a11y tests to all interactive components
+  - Test keyboard navigation flows
+
+### Child Branch 10: `feat/integration-testing`
+- **Issue**: No component interaction testing
+- **Actions**:
+  - Test version selection → diff loading → display flow
+  - Test diff completion → progress tracking integration
+  - Test theme switching across components
+  - Test responsive behavior integration
+
+### Child Branch 11: `feat/performance-testing`
+- **Issue**: No performance benchmarks or testing
+- **Actions**:
+  - Add performance testing utilities
+  - Test large diff rendering performance
+  - Add memory leak detection
+  - Create performance regression tests
+
+## Phase 5: CI/CD and Quality Gates (Branch: `testing-ci-quality`)
+
+### Child Branch 12: `feat/coverage-thresholds`
+- **Issue**: No enforced quality gates
+- **Actions**:
+  - Implement progressive coverage thresholds
+  - Add coverage requirements for new code
+  - Create coverage badges for README
+  - Set up coverage trend tracking
+
+### Child Branch 13: `feat/test-automation-improvements`
+- **Issue**: Limited CI testing scope
+- **Actions**:
+  - Add cross-browser e2e testing
+  - Implement parallel test execution
+  - Add test result reporting and notifications
+  - Create test flakiness detection
+
+## Success Metrics
+- **Component Coverage**: 7.7% → 85%+
+- **Hook Coverage**: 0% → 100%
+- **Util Coverage**: 33% → 90%+
+- **Overall Coverage**: ~15% → 80%+
+- **CI Pipeline**: Include all test types
+- **Quality Gates**: Enforce coverage thresholds
+
+## Timeline Estimate
+- **Phase 1**: 3-5 days (foundation critical)
+- **Phase 2**: 7-10 days (bulk of work)
+- **Phase 3**: 4-6 days (complex hook testing)
+- **Phase 4**: 5-7 days (advanced features)
+- **Phase 5**: 2-3 days (CI/automation)
+
+**Total**: ~3-4 weeks for comprehensive testing infrastructure