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

network.h

@@ -3,9 +3,115 @@
 
 #include <stdint.h>
 
+struct client;
+
+typedef enum {
+	PACKET_ERROR        = 0,
+	PACKET_REGISTER     = 1,
+	PACKET_AUTHENTICATE = 2,
+	PACKET_JOIN         = 3,
+	PACKET_TEXT         = 4,
+	PACKET_LEAVE        = 5,
+	PACKET_OK           = 6,
+	PACKET_DM_OPEN      = 7,
+	PACKET_DM_ROOM      = 8,
+	PACKET_CREATE_ROOM  = 9,
+	PACKET_DELETE_ROOM  = 10,
+	PACKET_ROOM_CREATED = 11,
+	PACKET_LIST_ROOMS   = 12,
+	PACKET_ROOM_LIST    = 13
+} packet_type;
+
+typedef enum {
+	ERR_OK                  = 0,
+	ERR_UNKNOWN             = 1,
+	ERR_INVALID_PACKET      = 2,
+	ERR_NOT_AUTHENTICATED   = 3,
+	ERR_ALREADY_REGISTERED  = 4,
+	ERR_INVALID_CREDENTIALS = 5,
+	ERR_REGISTRATION_DISABLED = 6,
+	ERR_DATABASE            = 7,
+	ERR_USER_NOT_FOUND      = 8,
+	ERR_ACCESS_DENIED       = 9,
+	ERR_ROOM_NOT_FOUND      = 10,
+	ERR_ROOM_NAME_TAKEN     = 11,
+	ERR_NOT_ROOM_OWNER      = 12
+} error_code;
+
 struct packet_header {
 	uint16_t size;
 	uint8_t type;
 };
 
+struct packet_register {
+	char username[20];
+	char password[100];
+};
+
+struct packet_auth {
+	char username[20];
+	char password[100];
+};
+
+struct packet_join {
+	uint64_t room_id;
+};
+
+struct packet_leave {
+	uint64_t room_id;
+};
+
+struct packet_text {
+	uint64_t room_id;
+	char message[];
+};
+
+struct packet_error {
+	uint8_t code;
+};
+
+struct packet_text_broadcast {
+	uint64_t room_id;
+	char username[32];
+	char message[];
+};
+
+struct packet_dm_open {
+	char username[20];
+};
+
+struct packet_dm_room {
+	uint64_t room_id;
+};
+
+struct packet_create_room {
+	char name[32];
+};
+
+struct packet_delete_room {
+	uint64_t room_id;
+};
+
+struct packet_room_created {
+	uint64_t room_id;
+	char name[32];
+};
+
+struct room_list_entry {
+	uint64_t room_id;
+	char name[32];
+	char owner[20];
+};
+
+struct packet_room_list {
+	uint32_t count;
+	struct room_list_entry rooms[];
+};
+
+int network_init(void);
+void network_shutdown(void);
+void network_handle_data(struct client *client, const char *data, size_t len);
+void network_client_add(struct client *client);
+void network_client_remove(struct client *client);
+
 #endif