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>

dev/cli/controller_cli.c

@@ -302,6 +302,39 @@ static void cmd_stop_ingest(struct Transport_Conn *conn,
 		proto_write_stop_ingest(conn, next_req_id(req), stream_id));
 }
 
+static void cmd_start_display(struct Transport_Conn *conn,
+	struct Ctrl_State *cs, uint16_t *req,
+	int ntok, char *tokens[])
+{
+	/* Required: stream_id
+	 * Optional: win_x win_y win_w win_h */
+	if (ntok < 2) {
+		printf("usage: start-display <stream_id> [win_x] [win_y] [win_w] [win_h]\n");
+		return;
+	}
+	uint16_t stream_id = (uint16_t)atoi(tokens[1]);
+	int16_t  win_x     = ntok > 2 ? (int16_t)atoi(tokens[2]) : 0;
+	int16_t  win_y     = ntok > 3 ? (int16_t)atoi(tokens[3]) : 0;
+	uint16_t win_w     = ntok > 4 ? (uint16_t)atoi(tokens[4]) : 0;
+	uint16_t win_h     = ntok > 5 ? (uint16_t)atoi(tokens[5]) : 0;
+	printf("start-display: stream=%u  pos=%d,%d  size=%ux%u\n",
+		stream_id, win_x, win_y, win_w, win_h);
+	SEND_AND_WAIT(cs, PROTO_CMD_START_DISPLAY,
+		proto_write_start_display(conn, next_req_id(req),
+			stream_id, win_x, win_y, win_w, win_h,
+			PROTO_DISPLAY_SCALE_FIT, PROTO_DISPLAY_ANCHOR_CENTER));
+}
+
+static void cmd_stop_display(struct Transport_Conn *conn,
+	struct Ctrl_State *cs, uint16_t *req,
+	const char *sid_str)
+{
+	uint16_t stream_id = (uint16_t)atoi(sid_str);
+	printf("stop-display: stream=%u\n", stream_id);
+	SEND_AND_WAIT(cs, PROTO_CMD_STOP_DISPLAY,
+		proto_write_stop_display(conn, next_req_id(req), stream_id));
+}
+
 static void cmd_help(void)
 {
 	printf("commands:\n"
@@ -312,6 +345,8 @@ static void cmd_help(void)
 		"  start-ingest <stream_id> <device> <dest_host> <dest_port>"
 			" [format] [width] [height] [fps_n] [fps_d]\n"
 		"  stop-ingest <stream_id>\n"
+		"  start-display <stream_id> [win_x] [win_y] [win_w] [win_h]\n"
+		"  stop-display <stream_id>\n"
 		"  help\n"
 		"  quit / exit\n");
 }
@@ -412,6 +447,11 @@ int main(int argc, char **argv)
 		} else if (strcmp(cmd, "stop-ingest") == 0) {
 			if (ntok < 2) { printf("usage: stop-ingest <stream_id>\n"); }
 			else { cmd_stop_ingest(conn, &cs, &req_id, tokens[1]); }
+		} else if (strcmp(cmd, "start-display") == 0) {
+			cmd_start_display(conn, &cs, &req_id, ntok, tokens);
+		} else if (strcmp(cmd, "stop-display") == 0) {
+			if (ntok < 2) { printf("usage: stop-display <stream_id>\n"); }
+			else { cmd_stop_display(conn, &cs, &req_id, tokens[1]); }
 		} else {
 			printf("unknown command: %s  (type 'help' for commands)\n", cmd);
 		}