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>

docs/protocol.md

@@ -491,6 +491,8 @@ packet-beta
 96-111: "win_h"
 112-119: "scale"
 120-127: "anchor"
+128-135: "no_signal_fps"
+136-143: "reserved"
 ```
 
 | Field | Description |
@@ -500,6 +502,7 @@ packet-beta
 | `win_w`, `win_h` | Window size in pixels; `0` = default (1280×720) |
 | `scale` | `0`=stretch `1`=fit `2`=fill `3`=1:1 |
 | `anchor` | `0`=center `1`=topleft |
+| `no_signal_fps` | Frame rate of no-signal animation (0 = default 15 fps) |
 
 **Response** — no extra fields beyond request_id and status. `OK` means the display slot was reserved; the window opens asynchronously on the main thread.