Add display sink: START_DISPLAY/STOP_DISPLAY, multi-window xorg, random port

Protocol:
- Add PROTO_CMD_START_DISPLAY (0x000A) and PROTO_CMD_STOP_DISPLAY (0x000B)
  with write/read functions; Proto_Start_Display carries stream_id, window
  position/size, scale and anchor; PROTO_DISPLAY_SCALE_*/ANCHOR_* constants

Node display sink:
- Display_Slot struct with wanted_state/current_state (DISP_CLOSED/DISP_OPEN);
  handlers set wanted state, display_loop_tick on main thread reconciles
- Up to MAX_DISPLAYS (4) simultaneous viewer windows
- on_frame routes incoming VIDEO_FRAME messages to matching display slot;
  transport thread deposits payload, main thread consumes without holding lock
  during JPEG decode/upload
- Main thread runs GL event loop when xorg is available; headless fallback
  joins reconciler timer thread as before

Xorg multi-window:
- Ref-count glfwInit/glfwTerminate via glfw_acquire/glfw_release so closing
  one viewer does not terminate GLFW for remaining windows
- Add glfwMakeContextCurrent before GL calls in push_yuv420, push_bgra,
  push_mjpeg and poll so each viewer uses its own GL context correctly

Transport random port:
- Bind port 0 lets the OS assign a free port; getsockname reads it back
  into server->bound_port after bind
- Add transport_server_get_port() accessor
- Default tcp_port changed from 8000 to 0 (random); node prints actual
  port after server start so it is always visible in output
- Add --port PORT CLI override (before config-file argument)

controller_cli:
- Add start-display and stop-display commands

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/modules/transport/transport.c

@@ -23,6 +23,7 @@ struct Transport_Conn {
 
 struct Transport_Server {
 	int                            listen_fd;
+	uint16_t                       bound_port;  /* actual port after bind */
 	struct Transport_Server_Config config;
 	pthread_t                      accept_thread;
 	pthread_mutex_t                count_mutex;
@@ -209,6 +210,15 @@ struct App_Error transport_server_start(struct Transport_Server *server) {
 		return APP_SYSCALL_ERROR();
 	}
 
+	/* Read back the actual port (matters when config.port == 0) */
+	struct sockaddr_in bound = {0};
+	socklen_t bound_len = sizeof(bound);
+	if (getsockname(fd, (struct sockaddr *)&bound, &bound_len) == 0) {
+		server->bound_port = ntohs(bound.sin_port);
+	} else {
+		server->bound_port = server->config.port;
+	}
+
 	if (listen(fd, SOMAXCONN) < 0) {
 		close(fd);
 		return APP_SYSCALL_ERROR();
@@ -235,6 +245,10 @@ void transport_server_destroy(struct Transport_Server *server) {
 	free(server);
 }
 
+uint16_t transport_server_get_port(const struct Transport_Server *server) {
+	return server->bound_port;
+}
+
 struct App_Error transport_connect(struct Transport_Conn **out,
 	const char *host, uint16_t port,
 	uint32_t max_payload,