fix: arbitrate display requests atomically

app/pigway_pi_control.py

@@ -488,6 +488,7 @@ class DisplayController:
         self.current_priority=0
         self.current_rgb_mode="OFF"
         self.owner_since=now
+        self.batch_depth=0
         self.stop_event=threading.Event()
         self.rgb_thread=threading.Thread(target=self._rgb_worker,name="rgb",daemon=True)
 
@@ -518,14 +519,23 @@ class DisplayController:
                             ("priority","severity","rgb_mode","title","l2","l3","l4","metadata"))
                 if not changed: request["last_changed"]=current["last_changed"]
                 self.requests[request_id]=request
-            self._arbitrate_locked()
+            if self.batch_depth==0: self._arbitrate_locked()
 
     def clear(self,request_id):
         if request_id=="NORMAL_HOME": return
         with self.lock:
             if self.requests.pop(request_id,None) is not None:
                 log("INFO","ALERT_RECOVERED",alert=request_id)
-                self._arbitrate_locked()
+                if self.batch_depth==0: self._arbitrate_locked()
+
+    def begin_update(self):
+        with self.lock: self.batch_depth+=1
+
+    def end_update(self):
+        with self.lock:
+            if self.batch_depth<=0: raise RuntimeError("display update batch imbalance")
+            self.batch_depth-=1
+            if self.batch_depth==0: self._arbitrate_locked()
 
     def _arbitrate_locked(self):
         current=self.requests.get(self.current_owner_id)
@@ -654,7 +664,9 @@ try:
 
             c,t,m,d=latest
             display.update_home(c,t,m,d)
-            update_active_alerts(c,t,m,d,sample_elapsed)
+            display.begin_update()
+            try: update_active_alerts(c,t,m,d,sample_elapsed)
+            finally: display.end_update()
             if now-last_render >= OLED_REFRESH:
                 display.refresh()
                 last_render=now