docs: Add sample data validation testing strategy

- Comprehensive testing strategy for cross-format validation
- Test utility helper class design (SampleDataValidator)
- Line number integrity and overlap detection
- External tool compatibility testing (diff, git diff)
- TempTestCases folder for ad-hoc testing
- Modular test structure with attribute-based discovery
- Worktree: feature/sample-data-validation-tests

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: randlee