Add unit test suite and complete protocol documentation

- 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

Makefile

@@ -30,8 +30,17 @@ users.h:
 asfur: ${OBJ}
 	${CC} -o $@ ${OBJ} ${LDFLAGS}
 
+test: test_network
+	./test_network
+
+test_network: test_network.o network.o password.o
+	${CC} -o $@ test_network.o network.o password.o ${LDFLAGS}
+
+test_network.o: test_network.c test.h network.h client.h password.h
+	${CC} -c ${CFLAGS} test_network.c
+
 clean:
-	rm -f asfur ${OBJ} asfur-${VERSION}.tar.gz
+	rm -f asfur test_network ${OBJ} test_network.o asfur-${VERSION}.tar.gz
 
 dist: clean
 	mkdir -p asfur-${VERSION}
@@ -53,4 +62,4 @@ uninstall:
 	rm -f ${DESTDIR}${PREFIX}/bin/asfur\
 		${DESTDIR}${MANPREFIX}/man1/asfur.1
 
-.PHONY: all options clean dist install uninstall
+.PHONY: all options clean dist install uninstall test