test: add comprehensive integration tests and physical device benchmarks

Integration Tests:
- End-to-end integration test suite (complete rendering pipeline)
- Mobile device integration tests (touch, screens, memory, platform)
- Cross-platform validation tests (iOS, Android, Desktop)

Physical Device Benchmarks:
- Comprehensive benchmark suite for physical devices
- Parse time benchmarks (1KB to 100KB)
- Memory profiling recommendations
- FPS measurement guidance
- Touch interaction latency
- Real-world scenario benchmarks

Documentation:
- TESTING_GUIDE.md with comprehensive testing instructions
- Platform-specific testing commands
- CI/CD integration examples
- Performance targets and best practices

Test Coverage Areas:
- Complete rendering pipeline validation
- Touch interactions (tap, long press, drag, pinch)
- Screen size variations (phone, tablet, orientation)
- Memory constraints and cleanup
- Platform-specific behaviors (iOS vs Android)
- Battery and resource optimization
- Error handling across platforms
- Complex layouts (floats, tables, nested elements)
- Real-world content (news, email, documentation)

All tests follow Flutter best practices and include detailed documentation.

Author: Nguyễn Tuấn Việt