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/protocol/protocol.c

@@ -601,6 +601,65 @@ struct App_Error proto_read_stop_ingest(
 	return APP_OK;
 }
 
+/* START_DISPLAY: request_id(2) cmd(2) stream_id(2) win_x(2) win_y(2)
+ * win_w(2) win_h(2) scale(1) anchor(1) = 16 bytes */
+struct App_Error proto_write_start_display(struct Transport_Conn *conn,
+	uint16_t request_id, uint16_t stream_id,
+	int16_t win_x, int16_t win_y, uint16_t win_w, uint16_t win_h,
+	uint8_t scale, uint8_t anchor)
+{
+	uint8_t buf[16];
+	uint32_t o = 0;
+	put_u16(buf, o, request_id);              o += 2;
+	put_u16(buf, o, PROTO_CMD_START_DISPLAY); o += 2;
+	put_u16(buf, o, stream_id);               o += 2;
+	put_i16(buf, o, win_x);                   o += 2;
+	put_i16(buf, o, win_y);                   o += 2;
+	put_u16(buf, o, win_w);                   o += 2;
+	put_u16(buf, o, win_h);                   o += 2;
+	put_u8 (buf, o, scale);                   o += 1;
+	put_u8 (buf, o, anchor);                  o += 1;
+	(void)o;
+	return transport_send_frame(conn, PROTO_MSG_CONTROL_REQUEST, buf, 16);
+}
+
+struct App_Error proto_write_stop_display(struct Transport_Conn *conn,
+	uint16_t request_id, uint16_t stream_id)
+{
+	uint8_t buf[6];
+	put_u16(buf, 0, request_id);
+	put_u16(buf, 2, PROTO_CMD_STOP_DISPLAY);
+	put_u16(buf, 4, stream_id);
+	return transport_send_frame(conn, PROTO_MSG_CONTROL_REQUEST, buf, 6);
+}
+
+struct App_Error proto_read_start_display(
+	const uint8_t *payload, uint32_t length,
+	struct Proto_Start_Display *out)
+{
+	if (length < 16) { return APP_INVALID_ERROR_MSG(0, "START_DISPLAY payload too short"); }
+	out->request_id = get_u16(payload,  0);
+	/* skip command word at [2..3] */
+	out->stream_id  = get_u16(payload,  4);
+	out->win_x      = get_i16(payload,  6);
+	out->win_y      = get_i16(payload,  8);
+	out->win_w      = get_u16(payload, 10);
+	out->win_h      = get_u16(payload, 12);
+	out->scale      = get_u8 (payload, 14);
+	out->anchor     = get_u8 (payload, 15);
+	return APP_OK;
+}
+
+struct App_Error proto_read_stop_display(
+	const uint8_t *payload, uint32_t length,
+	struct Proto_Stop_Display *out)
+{
+	if (length < 6) { return APP_INVALID_ERROR_MSG(0, "STOP_DISPLAY payload too short"); }
+	out->request_id = get_u16(payload, 0);
+	out->stream_id  = get_u16(payload, 4);
+	return APP_OK;
+}
+
 struct App_Error proto_read_response_header(
 	const uint8_t *payload, uint32_t length,
 	struct Proto_Response_Header *out)

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,

src/modules/xorg/xorg.c

@@ -12,6 +12,23 @@
 #include "xorg.h"
 #include "font_atlas.h"   /* generated: font_glyphs[], font_atlas_pixels[], FONT_ATLAS_W/H */
 
+/* Reference count for glfwInit/glfwTerminate.
+ * All xorg calls happen on the main thread, so no locking needed. */
+static int glfw_ref_count = 0;
+
+static void glfw_acquire(void)
+{
+	if (glfw_ref_count == 0) { glfwInit(); }
+	glfw_ref_count++;
+}
+
+static void glfw_release(void)
+{
+	if (glfw_ref_count <= 0) { return; }
+	glfw_ref_count--;
+	if (glfw_ref_count == 0) { glfwTerminate(); }
+}
+
 /* ------------------------------------------------------------------ */
 /* Shader sources — video                                              */
 /* ------------------------------------------------------------------ */
@@ -326,10 +343,7 @@ static bool init_text_rendering(Xorg_Viewer *v)
 Xorg_Viewer *xorg_viewer_open(int x, int y, int width, int height,
                                const char *title)
 {
-	if (!glfwInit()) {
-		fprintf(stderr, "xorg: glfwInit failed\n");
-		return NULL;
-	}
+	glfw_acquire();
 
 	glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
 	glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
@@ -338,7 +352,7 @@ Xorg_Viewer *xorg_viewer_open(int x, int y, int width, int height,
 	GLFWwindow *win = glfwCreateWindow(width, height, title, NULL, NULL);
 	if (!win) {
 		fprintf(stderr, "xorg: glfwCreateWindow failed\n");
-		glfwTerminate();
+		glfw_release();
 		return NULL;
 	}
 	glfwSetWindowPos(win, x, y);
@@ -350,14 +364,14 @@ Xorg_Viewer *xorg_viewer_open(int x, int y, int width, int height,
 	if (glewInit() != GLEW_OK) {
 		fprintf(stderr, "xorg: glewInit failed\n");
 		glfwDestroyWindow(win);
-		glfwTerminate();
+		glfw_release();
 		return NULL;
 	}
 
 	Xorg_Viewer *v = calloc(1, sizeof(*v));
 	if (!v) {
 		glfwDestroyWindow(win);
-		glfwTerminate();
+		glfw_release();
 		return NULL;
 	}
 	v->window = win;
@@ -727,6 +741,7 @@ bool xorg_viewer_push_yuv420(Xorg_Viewer *v,
                               int width, int height)
 {
 	if (!v) { return false; }
+	glfwMakeContextCurrent(v->window);
 	v->frame_w = width;
 	v->frame_h = height;
 	upload_yuv(v, y, width, height, cb, width / 2, height / 2, cr);
@@ -737,6 +752,7 @@ bool xorg_viewer_push_bgra(Xorg_Viewer *v,
                             const uint8_t *data, int width, int height)
 {
 	if (!v) { return false; }
+	glfwMakeContextCurrent(v->window);
 
 	v->frame_w = width;
 	v->frame_h = height;
@@ -759,6 +775,7 @@ bool xorg_viewer_push_mjpeg(Xorg_Viewer *v,
 	return false;
 #else
 	if (!v) { return false; }
+	glfwMakeContextCurrent(v->window);
 
 	int w, h, subsamp, colorspace;
 	if (tjDecompressHeader3(v->tj, data, (unsigned long)size,
@@ -809,6 +826,7 @@ bool xorg_viewer_poll(Xorg_Viewer *v)
 	if (!v || glfwWindowShouldClose(v->window)) { return false; }
 	glfwPollEvents();
 	if (glfwWindowShouldClose(v->window)) { return false; }
+	glfwMakeContextCurrent(v->window);
 	render(v);
 	return true;
 }
@@ -838,7 +856,7 @@ void xorg_viewer_close(Xorg_Viewer *v)
 	if (v->prog_rgb)  { glDeleteProgram(v->prog_rgb); }
 	if (v->window) {
 		glfwDestroyWindow(v->window);
-		glfwTerminate();
+		glfw_release();
 	}
 	free(v);
 }