- Add test framework (test.h) with assertion macros and test runner
- Add comprehensive unit tests (test_network.c) covering:
- Packet parsing and error handling
- User registration and authentication
- Room operations (create, join, leave, delete, list)
- Direct messaging functionality
- Message broadcasting
- Password hashing
- Update Makefile with 'make test' target
- Rewrite PROTOCOL file with complete specification:
- All 14 packet types with data layouts
- All error codes with descriptions
- Typical usage flows
