fix: use REQUEST (type 0) + drain response after each send

RouterBridge on MCU ignores NOTIFICATION (type 2) messages.
Must use REQUEST (type 0) for draw_frame, then drain the
response to prevent router buffer buildup.

scripts/led-matrix-viz.py

@@ -74,6 +74,8 @@ def main():
 
     _poll.register(sock, POLLIN)
 
+    router_resp = b""
+
     active_notes = set()
     last_sent_params = None
     pending_bytes = None
@@ -134,7 +136,7 @@ def main():
                             params = render(active_notes)
                             if params != last_sent_params:
                                 msg_id = (msg_id + 1) % 65535
-                                pending_bytes = packer.pack([2, msg_id, "draw_frame", params])
+                                pending_bytes = packer.pack([0, msg_id, "draw_frame", params])
                                 last_sent_params = params
 
         # Try to send pending frame — no POLLOUT dependency
@@ -143,6 +145,14 @@ def main():
             if ok:
                 pending_bytes = None
                 frames_sent += 1
+                # Drain exactly one response to keep router buffer clear
+                try:
+                    router.settimeout(0.001)
+                    router.recv(4096)
+                except (BlockingIOError, socket.timeout, OSError):
+                    pass
+                finally:
+                    router.settimeout(0.01)
             else:
                 frames_dropped += 1
 
@@ -152,7 +162,7 @@ def main():
             fps_sample_frames += 1
             if last_sent_params is not None and pending_bytes is None:
                 msg_id = (msg_id + 1) % 65535
-                refresh_bytes = packer.pack([2, msg_id, "draw_frame", last_sent_params])
+                refresh_bytes = packer.pack([0, msg_id, "draw_frame", last_sent_params])
                 router, ok = try_send(router, refresh_bytes)
                 if ok:
                     frames_sent += 1