Add no-signal animation to display windows

When a viewer window has no incoming stream, renders animated analog-TV
noise (hash-based, scanlines, phosphor tint) at configurable fps (default
15) with a centred "NO SIGNAL" text overlay.

- xorg: FRAG_NOSIGNAL_SRC shader + xorg_viewer_render_no_signal(v, time, noise_res)
- main: Display_Slot gains no_signal_fps + last_no_signal_t; display_loop_tick
  drives no-signal render on idle slots via clock_gettime rate limiting
- protocol: START_DISPLAY extended by 2 bytes — no_signal_fps (0=default 15)
  + reserved; reader is backward-compatible (defaults 0 if length < 18)
- controller_cli: no_signal_fps optional arg on start-display
- docs: protocol.md updated with new field

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

include/protocol.h

@@ -251,6 +251,8 @@ struct Proto_Start_Display {
 	uint16_t win_h;
 	uint8_t  scale;
 	uint8_t  anchor;
+	uint8_t  no_signal_fps;  /* 0 = default (15); no-signal animation frame rate */
+	/* 1 byte reserved */
 };
 
 struct Proto_Stop_Display {
@@ -340,7 +342,7 @@ struct App_Error proto_write_stop_ingest(struct Transport_Conn *conn,
 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 scale, uint8_t anchor, uint8_t no_signal_fps);
 
 /* CONTROL_REQUEST: STOP_DISPLAY */
 struct App_Error proto_write_stop_display(struct Transport_Conn *conn,