C SDK changes:
- Add #include un.h when building as library
- Export library functions: version, detect_language, hmac_sign, health_check
- Add NULL safety checks to wrapper functions
- Rewrite tests to call ACTUAL exported functions (no more local re-implementations)
- Tests now verify real HMAC-SHA256 against known test vectors
Documentation:
- Add SDK Testing Philosophy to CLAUDE.md
- Create docs/TESTING.md with unit/integration/functional test definitions
- Document the THREE testing levels required for all SDKs
- Explicitly forbid mocking and local re-implementation in tests
33 unit tests pass, testing real exported functions.
