Implement Functional Output Validation Framework

[krystophny]

Summary

Implement mandatory functional validation requirements to prevent systemic failures where core plotting functionality breaks without detection, as described in Issue #92.

Background

Recent issues revealed that unit tests were passing while plot generation was completely broken, leading to multiple PRs merging without functional validation and documentation examples becoming non-functional.

Requirements

Quality Gates

• Unit tests must generate actual plot files - No mocking file I/O for plot generation
• Examples must execute and produce outputs - All example/ programs must run successfully
• Generated files must be valid - File size > 0 bytes and proper format headers
• Backend compatibility - PNG/PDF backends must produce viewable outputs
• ASCII backend functionality - ASCII plots must display properly in terminal

Test Framework Enhancements

• Extend TDD workflow with output file generation verification
• Create functional tests in test/ directory that generate actual plot outputs
• Implement output file verification (existence and non-empty validation)
• Add backend testing for all supported formats (PNG, PDF, ASCII)
• Establish baseline comparison system for regression testing

CI/Build Integration

• Integrate functional validation into make targets
• Add output verification to build process
• Create validation scripts for generated files
• Ensure all examples execute successfully in CI

Agent Role Integration

• georg-test-engineer: Implement functional test creation with actual plot output generation
• vicky-acceptance-tester: Execute and validate all examples, inspect output quality
• patrick-auditor: Review output validation coverage, verify regression prevention
• sergei-perfectionist-coder: Implement proper error handling and output generation

Implementation Strategy

1. Phase 1: Update existing unit tests to generate actual plot files
2. Phase 2: Create functional validation test suite
3. Phase 3: Integrate validation into CI pipeline
4. Phase 4: Establish baseline comparison framework
5. Phase 5: Documentation and workflow integration

Acceptance Criteria

• All tests generate actual plot outputs (no mocked I/O)
• All examples in example/ directory execute successfully
• Generated plot files are valid and non-empty
• All backends (PNG, PDF, ASCII) produce correct outputs
• CI pipeline fails if plot generation fails
• Reviewers have clear checklist for output validation

Priority

HIGH - Critical for preventing future systemic failures and maintaining library functionality.

Labels

• enhancement
• documentation

[krystophny]

Functional validation framework successfully implemented and merged in PR #112.

COMPLETED DELIVERABLES:

✅ Functional validation interface with pytest-like API

• type for structured validation results
• , , , ,
• for regression detection

✅ Comprehensive test coverage protecting all plotting functionality

• • Core plotting validation
• • Edge case protection
• Enhanced PDF content regression tests

✅ Enhanced build system integration

• New make target for functional testing
• CI workflow improvements for test file handling
• Added validation interface to public fortplot API

✅ Architectural documentation

• Complete validation framework specification in DESIGN.md
• Integration patterns and usage examples
• Quality gate enforcement protocols

The functional validation framework is now protecting all plotting functionality with comprehensive test coverage, enabling confident refactoring and feature development.