feat: speed limit ding sound when cruise warning sign appears

Plays a ding via soundd when the cruise warning sign becomes visible
(cruise set speed out of range vs speed limit) or when the speed limit
changes while the warning sign is already showing. Max 1 ding per 30s.

Ding is mixed independently into soundd output at max volume without
interrupting alert sounds. bench_cmd ding available for manual trigger.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

selfdrive/clearpilot/bench_cmd.py

@@ -9,6 +9,7 @@ Usage:
   python3 -m selfdrive.clearpilot.bench_cmd cruise 55
   python3 -m selfdrive.clearpilot.bench_cmd cruiseactive 0|1|2  (0=disabled, 1=active, 2=paused)
   python3 -m selfdrive.clearpilot.bench_cmd engaged 1
+  python3 -m selfdrive.clearpilot.bench_cmd ding              (trigger speed limit ding sound)
   python3 -m selfdrive.clearpilot.bench_cmd debugbutton       (simulate LKAS debug button press)
   python3 -m selfdrive.clearpilot.bench_cmd dump
   python3 -m selfdrive.clearpilot.bench_cmd wait_ready
@@ -108,6 +109,10 @@ def main():
   elif cmd == "wait_ready":
     wait_ready()
 
+  elif cmd == "ding":
+    params.put("ClearpilotPlayDing", "1")
+    print("Ding triggered")
+
   elif cmd == "debugbutton":
     # Simulate LKAS debug button — same state machine as controlsd.clearpilot_state_control()
     current = params.get_int("ScreenDisplayMode")