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>

include/transport.h

@@ -51,9 +51,15 @@ struct Transport_Server_Config {
 struct App_Error  transport_server_create(struct Transport_Server **out,
 	struct Transport_Server_Config *config);
 
-/* Bind, listen, and spawn the accept thread. */
+/* Bind, listen, and spawn the accept thread.
+ * If config.port is 0, the OS assigns a free port; use
+ * transport_server_get_port() afterwards to retrieve it. */
 struct App_Error  transport_server_start(struct Transport_Server *server);
 
+/* Return the port the server is actually listening on.
+ * Valid after a successful transport_server_start(). */
+uint16_t          transport_server_get_port(const struct Transport_Server *server);
+
 /*
  * Stop accepting new connections and free the server.
  * Active connections continue until they disconnect naturally.