Add comprehensive testing infrastructure for UN clients:
1. Smart Change Detection (detect-changes.sh)
- Detects changes in BOTH root-level (un.py, un.go) AND clients/ directory
- Maps file extensions and directory names to languages
- Triggers test_all when infrastructure changes
2. Language-Specific CI Matrix (generate-matrix.sh compatible)
- Only runs tests for languages with changes
- Example: modify clients/python/ → pytest runs, Go/Ruby skipped
3. Testing Strategy Document (TESTING-STRATEGY.md)
- Complete testing matrix by language tier (compiled, interpreted, inception)
- Unit, integration, embedding, and parity tests
- Inception pattern for languages without local interpreters
- Common failures and fixes
- Rollout schedule for client/ migration
4. Makefile Targets
- 'make test-python', 'make test-go', etc. for local development
- 'make test-all' for comprehensive testing
- 'make test-integration' for API contract validation
- 'make test-ci-locally' to simulate CI pipeline
5. Updated CLAUDE.md
- Documents SDK architecture (in growth state)
- Explains three purposes: CLI, library, embeddable
- References TESTING-STRATEGY.md for details
This enables:
✓ Per-language testing (only run what changed)
✓ Local developer workflow (make test-LANG)
✓ 42+ language feature parity validation
✓ Cross-language integration testing
