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/protocol.h

@@ -26,6 +26,8 @@
 #define PROTO_CMD_ENUM_MONITORS      0x0007u
 #define PROTO_CMD_START_INGEST       0x0008u
 #define PROTO_CMD_STOP_INGEST        0x0009u
+#define PROTO_CMD_START_DISPLAY      0x000Au
+#define PROTO_CMD_STOP_DISPLAY       0x000Bu
 
 /* -------------------------------------------------------------------------
  * Response status codes (carried in CONTROL_RESPONSE payload offset 2)
@@ -232,6 +234,39 @@ struct Proto_Stop_Ingest {
 	uint16_t stream_id;
 };
 
+/*
+ * START_DISPLAY: controller tells a sink node to open a viewer window and
+ * display incoming VIDEO_FRAME messages for the given stream_id.
+ * win_x/win_y are screen-space window position (signed: multi-monitor).
+ * win_w/win_h of 0 mean use a default size.
+ * scale: 0=stretch 1=fit 2=fill 3=1:1 (PROTO_DISPLAY_SCALE_*)
+ * anchor: 0=center 1=topleft (PROTO_DISPLAY_ANCHOR_*)
+ */
+struct Proto_Start_Display {
+	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;
+};
+
+struct Proto_Stop_Display {
+	uint16_t request_id;
+	uint16_t stream_id;
+};
+
+/* Scale/anchor constants for Proto_Start_Display */
+#define PROTO_DISPLAY_SCALE_STRETCH  0u
+#define PROTO_DISPLAY_SCALE_FIT      1u
+#define PROTO_DISPLAY_SCALE_FILL     2u
+#define PROTO_DISPLAY_SCALE_1_1      3u
+
+#define PROTO_DISPLAY_ANCHOR_CENTER   0u
+#define PROTO_DISPLAY_ANCHOR_TOPLEFT  1u
+
 struct Proto_Response_Header {
 	uint16_t request_id;
 	uint16_t status;
@@ -301,6 +336,16 @@ struct App_Error proto_write_start_ingest(struct Transport_Conn *conn,
 struct App_Error proto_write_stop_ingest(struct Transport_Conn *conn,
 	uint16_t request_id, uint16_t stream_id);
 
+/* CONTROL_REQUEST: START_DISPLAY */
+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);
+
+/* CONTROL_REQUEST: STOP_DISPLAY */
+struct App_Error proto_write_stop_display(struct Transport_Conn *conn,
+	uint16_t request_id, uint16_t stream_id);
+
 /*
  * CONTROL_RESPONSE: generic.
  * payload/payload_len are the command-specific bytes after request_id+status.
@@ -381,6 +426,14 @@ struct App_Error proto_read_stop_ingest(
 	const uint8_t *payload, uint32_t length,
 	struct Proto_Stop_Ingest *out);
 
+struct App_Error proto_read_start_display(
+	const uint8_t *payload, uint32_t length,
+	struct Proto_Start_Display *out);
+
+struct App_Error proto_read_stop_display(
+	const uint8_t *payload, uint32_t length,
+	struct Proto_Stop_Display *out);
+
 /*
  * Read the common 4-byte response header (request_id + status).
  * For responses with no extra fields (STREAM_OPEN, STREAM_CLOSE, SET_CONTROL),