add viz diagnostic metrics to prometheus-exporter

- Viz writes stats to /tmp/viz-stats.json (frames_sent, frames_dropped, queue_depth)
- Exporter reads viz-stats.json and exposes:
  - viz_frames_sent_total (counter)
  - viz_frames_dropped_total (counter)
  - viz_pending_queue (gauge)
  - viz_queue_full (gauge)

scripts/led-matrix-viz.py

@@ -15,6 +15,7 @@ ROWS = 8
 frame_interval = 1.0 / 30.0  # 30 FPS target
 next_frame_time = 0.0
 
+STATS_FILE = "/tmp/viz-stats.json"
 _poll = select.poll()
 POLLIN = select.POLLIN
 POLLOUT = select.POLLOUT
@@ -191,6 +192,29 @@ def main():
                     frames_dropped += 1
                 last_frame = frame
 
+            # Write stats periodically (every 2s)
+            if now - _stats_last_write >= 2.0:
+                _write_stats(frames_sent, frames_dropped, pending_frames)
+                _stats_last_write = now
+
+
+_stats_last_write = 0.0
+
+
+def _write_stats(sent, dropped, pending):
+    import json
+    try:
+        with open(STATS_FILE, "w") as f:
+            json.dump({
+                "frames_sent": sent,
+                "frames_dropped": dropped,
+                "pending_queue": len(pending),
+                "queue_full": pending.full(),
+                "ts": time.time()
+            }, f)
+    except OSError:
+        pass
+
 
 if __name__ == "__main__":
     main()